Ive done this before with mice, hamsters and rats but i dont know how well it will work for sugar gliders. Someone can tell you if its safe or not. You take a piece of glass (we always used the glass from picture frames) and tape the edges with duct tape if its sharp. Put the animal on the glass and give it a treat, licky treats like yogurt and honey work best so they dont sit up to eat. You can lift the glass and look underneath or take a photo to determine what sex. I recommend having another person there to hold the glass while you look.
Protip: make sure the glass is clean.