How do you recognize a high Se person irl?
To spot any type or function in general, I'd say you need to keep into account every other functions at some point. For High Se, you need to keep an eye for low Ni, and also the presence of High Ji.
I will mainly talk from my experience, so keep in mind my own type to warn yourself from my type's biases.
At first, High Se can be confounded with most types, especially to an untrained eye. High Se users jumps on opportunities, they easily go into action in the sense of doing concrete stuff. Be it going for a walk on a whim, doing some sport, deciding to start a project, going outside and doing machinery, or any kind of physical labor. But there are many types that can easily act on almost right away on opportunities or create those opportunities even. So, the lines get blurry there.
With High Se, what needs to be taken into account is also the High Ji. Depending on the placement, the goal of either the Ji or Se will change, but in any case Se usually makes them opportunity takers and people that easily go into action. On the other hand, High Ji will incline to the sort of activities Se will jump on. High Ti will be more inclined to patenting and puzzling stuff, to get a grasp on how "something" or someone even works. There is a desire to systemize and jump on opportunities to test out their comprehension. And high Fi will incline to activities with some sort of personal value, be it an artistic project, a passion, or also some patenting stuff but usually with a moral goal behind. It is less in a goal to test in their case and more to do what they personally like or that strongly matters to them. Therefore, High Se users usually jump on occasions to deepen their Ti or Fi, so it is essential to spot the desire to act upon a personal goal, something less apparent at first but very present in the action and opportunity they choose to chase. If there is no interest for the High Ji, I can assure you Se won't give a single shit as it won't perceive the opportunity as one.
I will contrast with some types to better point out how High Se comes out contrary to others.
SJs might look like a High Se at first, especially ESxJ, since they won't hesitate to act upon stuff directly either. But what differentiates High Se from an SJ is how they act spontaneously or on a whim on what presents itself to them while SJs actually have another "larger" goal in mind. As I have noticed, SJs will act quite rapidly or be into action easily, but there is still some form of planning done before doing the said stuff. They don't necessarily jump right on, since they will first judge through Fe and Te if it answers their Je goals, and with Si perceives the desired and expected outcome and so what to do concretely to make it happen. It asks some time, there is a slight delay before they actually do something. Sometimes it is a mere matters of minutes, but in order to plan out on a grander scale what will need to be done. However, that planning, even if done very quickly, can be noticed contrary to High Se users where this planning seems totally non existent. High Se acts without thinking about the consequences much, it acts then judge from the new results what else to do. It is in part due to low Ni. When healthy and older, they use it to ensure the consequences are not meant to be absolutely disastrous, but that's about it. So, when it comes to real life, High Se spontaneously do it. They also easily shifts their goals and actually actualize it in the moment while SJs have somehow planned their actions.
I know it seems pretty ambiguous, so I will use an example.
Ex.: For context, I work as a social worker for a therapeutic center with people stuck with addiction problems (drugs, alscohol, gambling, etc.). I have both an ESFP and ESFJ coworker that I work often with for evening shifts, which asks mainly to watch over the center's clients and respond to their demands. Both at first are really prone to action and act rapidly on whatever is demanded upon us, while I am slow as a turtle and act when everything is settled in my mind (which asks me a day before work to actually think about this day and what I'll likely need to do). However, after working with them for some months, it became very obvious which one had Se and the other Si. My ESFP coworker, whatever we are doing, be it a friendly talk, if a client asks him if he wants to play some sport (since we have a gym), he would be jumping on this opportunity right on since it is something with more value to his Fi, and also because it helps him reinforce his bonds with our clients which is part of our job. So, this also calls on his Te. There is no after thought, he just stops talking, direct his attention on the client, and just join them right away. The conversation ends here and also, we never really talk about that subject ever again. While when something similar happened with my ESFJ coworker, they first delegated me stuff to do or what else will need to be done and then went playing with our clients. Also, they actually continued the conversation at some point.
My ESFP coworker can also ask how we will delegate the tasks, but they won't decide for me like the ESFJ tends to do. They might ask to do some tasks because of some Fi reasons while my ESFJ coworker says they don't have any preference, and if they do, they tend not to verbalize it and either don't debate it much or arrange the planning around that single preference. Also, if something unexpected happens, they tend to react very angrily (they are likely a 2w1, so the w1 shows) in order to push things to align with what they planned. While my ESFP coworker might complain but they adapt without a fuss to the circumstances. Their Te usually helps a lot to still maintain the order but with way more flexibility than I or the ESFJ's Je/Pi. ESFP is more lax with rules, while the ESFJ is deemed strict by the clients sometimes. However the tertiary Te and Ne of each helps them be balanced. While I am either too lax or too strict with rules.
So, if there is a resistance to the events, to the unexpected cirumstances, it is more likely an SJ than an SP, as the latter adapts themselves to the events while the former wants to adapt the circumstances to their vision (this also applies for NPs and NJs).
So, as you can see, despite the ESFJ being very active and still acting on quickly, there is always a planning that can be noticed through their way of delagating, getting frustrated and becoming even more strict due to unexpected events, or arranging in a more impersonal Je (Fe-Ti) way what needs to be done. While my ESFP coworker adapts on the moment, and while taking some time to see what needs to be done, they usually do what needs to be done without asking me or delagating anything as they just react on demands as they come.
It would be similar with ENxJs, their tert Se might make it harder at first, but the planification can be noticed and there would be some delay or some thought out plan in their actions. They would automatically be less spontaneous than an SP even if more comfortable with unpredictable demand than an INxJ.
And about High Ne? Well, honestly, that's pretty simple imo. High Ne acts hardly on their ideas even if they have tons of them. They are more talkers than acters, while High Se are the reversed; act more than talk. High Ne jumps on every ideas, so it is said but everythinig is yet to be done. It is mainly conceptual and barely tangible. While High Se acts on opportunnities, which means on the moment circumstances. I have already made a long post about this though, so I suggest searching it if you want more details about what differentiates High Se and High Ne.
Also, lastly, ISxPs might make it ambiguous at first too, since they act according to their Ji first and also their Ni is more highly regarded than with ESxPs. They won't be as much reactive and their Se will depend a lot on their Ji. So, they might not be as thoughtless in their actions, but usually they act on their goals or Ji demands very easily compared to their INxJ counterparts, which are very slooooow and very dependent on their auxiliary healthiness to do anythhing ever. But they can be said to be a more though out version of their extraverted counter parts but also less into action. Though, everything said above should apply at some point. They are however more balanced in their perception, making their Se more thought out and considerate for their actions's consequences (unless looping).
I hope it could help. I am not the best for thinking about concrete examples (*inf Se*), so neither to eleaborate the concrete details of the function's manfestations. I also tried to say the essential, but there are many more things that could be said about it. Also, keep in mind the enneagram types, it strongly influences Se's manifestation (ex.: High Se x 7 vs High Se x 9).