Hey, um, wow, I'm so nervous, but, um, how can I tell the difference between males and females in animals that don't have gender-specific traits, like the male lion has a mane, or a female proboscis monkey has a smaller nose?
Behavior, usually! Even without sexual dimorphism- or monomorphism, to be more precise -animals have marked differences in other ways. Take some species of finches, for example. While both females and males have their different chirps, it's the males that generally have longer and more involved song and dance routines. Females will also display nesting behavior, laying eggs even in the absence of a male; heh, that's the biggest giveaway right there!
It can be difficult when observing creatures like this, but usually if you watch long enough, you start to pick up key differences between them, even if you can't look and instantly tell. Unfortunately this isn't always a hard and fast rule, and some animals need to be professionally sexed via methods like ultrasound and laparoscopic examination. Buuut those are usually only important in breeding programs and other establishments, so hopefully even if you can't tell if a creature is male or female, that won't diminish your enjoyment of watching it any!