I just bought two sugar gliders, and I can tell easily that one is a boy, but I'm not sure about the other one. The glider won't let me hold them upside down, so I can't check that way, and I'm not quite sure if I see bald spots on the head or not.

Have you tired looking at the glider while its on the side of the cage? How come you didnt ask the breeder or petshop what sex they were?

I would say the side of the cage is best too. Give them treats from outside the cage so they have to climb on it. If they're side by side, it will much easier to see if their "anatomy" is the same.

The person that sold them to me lied fro some inexplicable reason about their gender. They told me that it was a mother and her son, when I just found out that the bigger glider is a male, and now I'm trying to figure out if the smaller one really is a male.

ugh I'm sorry to hear that :( if they are related you should have the male neutered.

do either of them have a bald spot? That's usually a good sign.

My suggestion would be to take them to the vet for a well check.
They need to go and have fecals done, if you say the breeder was dishonest in what they sold you, they could have not disclosed illness or issues these little ones may have.
The vet will also be able to tell you what gender they are.
Good luck with your little ones. :)

Yeah that side of the cage is good but my males bald spot on his head is really noticable! The lady didnt lie to me but if somebody would have it would have been very easy for me to tell that he was a male lol