The extension was never meant to replace (or shouldn't have been) checking for yourself. The purpose of a flagging system, especially one where the USERS are who flags, is to say "hey, this person might be a big transphobe, take a look."
So is there a more reliable flagging system? No, but that just means double-checking the perspective as much as possible.
There are more nuanced deep-dives into TERFism, radfeminism, and "general" transphobia respectively but some things to look out for for all
Use of slurs or disparaging language directed at a specific group of people in a derogatory sense. Do they use slurs not to reclaim (not assessing whether they are "allowed" to reclaim) but to be mean? There's that famous comic strip about how even mundane words like "pepperoni" being said in a way to express disdain hits in a way that reclamation doesn't. ie "I'm queer" "the queer community" "I love you queers" vs "ugh the queers are ruining everything." A lot of these groups will insist their use of slurs and derogatory language is "punching up" or "getting back at" groups who have wronged them, but every actual slur carries the weight of marginalization, and two oppressed groups weaponizing slurs at one another isn't "punching up."
Insistence on separatism. Separatist movements are ones in which a group who has experienced wrongs either systemically or inter-personally (or both) finds their solution for protection is isolation from groups that are not identical to them. It makes logical sense to think that people who are not like you might be unsafe and untrustworthy, but this is incorrect. The idea that there are innate differences between groups that cannot ever be overcome is the basis for everything from incel groups to terfs to neo-radfems. The problem is that at the extreme end, the separatism stems from a belief of superiority, and at the "milder" end, it causes people to heavily police who is and isn't enough to belong, inevitably isolating people who even by their own metric "should" belong.
Essentialism. This is the belief that there are inherent traits, behaviors, experiences that cannot be shared among various groups of people, and that our identities are set in stone by either birth or identity. TERFs believe that being assigned male at birth is a situation that can never be undone. That the parts you were born with determines who you are, and no amount of hormones or surgery or experiences changes that, and that the set in stone identity is "predator." They think, either literally or for the sake of propaganda, that men and anyone "born male" is destined to be a predator, violent, and essentially a threat. Radfems believe that maleness as a construct is violence, creates predator behavior, etc and so your proximity to such a concept determines your evil. Misogynists believe that being assigned female or being female makes you weak, inferior, untrustworthy, etc. Two camps of racist thought say that either being born not-white makes you inferior (either weak or a predator, situation dependent) while others believe that you can achieve proximity to whiteness by how much you can distance yourself from your race and/or contribute to the subjugation of other "inferior" types of people. White supremacist orgs love having men of color in their secondary leadership, not because they have overcome their racism, but because it creates a relationship where the POC fight harder for approval among their nazi peers than other white men do, and they are much more easily disposable if it comes to it--while in the meantime being held up as "proof" that the org cannot be racist.
A deep interest in restricting self-determination. Transphobes love having endless debates about "what is a woman," not for any philosophical sense, but because they think that "woman" is an innate category one can neither escape from nor move to. "Womanhood" is simultaneously something so retched it's the only reason trans men transition--to escape it--but is also something so sacred and coveted that trans women are trying to steal it. Allowing people to define womanhood on their own terms is not allowed. Allowing people to call themselves what fits them best is not allowed. Allowing people to alter their bodies and appearance and how people refer to them is not allowed. It is essentialism again, but it is also the paradoxical beliefs that things are inherent but that you must also work hard to conform. A "woman" is xyz, but if you do not look or act like that, you are forced to change to fit this vision. Women who don't wear makeup are punished for it. Makeup is not an "inherent" trait--no one is born with winged liner--but if a desire to wear makeup is not part of who you are, you must "fix it." Every time we attempt to create hard borders between what is and what isn't an identity, we leave people out. If a woman is girly and loves pink, then every woman (cis, trans, or other) who isn't very feminine or doesn't like pink "ceases to be" a woman until she "corrects" it. If being a trans woman is about your ability to pass as cis, everyone who doesn't pass enough isn't a woman--or isn't a trans woman, depending on which separatist group is watching. Turning definitions into static identities we must adhere to rather than describing a pattern of similar traits will always create gaps where even people we never intend to exclude are left out. This isn't just cis vs trans, either. Cis men who fail to perform masculinity efficiently enough are "like a girl," women of color have to overperform femininity to not be seen as "masculine" or "male"--look at Serena Williams or Michelle Obama: two very feminine women who are never afforded the belief that their Black femininity is feminine. Trans folks are guilty of it too. There is the expectation that trans men must try harder to prove their maleness, but then are punished more severely for toxic masculinity than their cis peers. Sometimes trans women, like their cis peers, brutally punish gender-nonconformity in other trans women, holding up stereotypical cis femininity as not only the epitome, but the only way to be.
This is not a comprehensive list, but a lot of repeated patterns to look out for. It is important to recognize that trans people can be transphobic, marginalized people can throw others under the bus for an attempt at integration into the majority, and that trying to control how any marginalized group tries to make sense of their situation is not "striking back" at your oppressors. But it is also important to know that there are no inherent impassable differences between groups of people, and the ones who bridge the gaps are doing more good for our communities' safety than anyone who tries to lock the doors between us.
