Healthy glider poop looks very much like mouse poop. It should be dark in color but can change color depending on what they eat.
If the glider is constipated, you will hear hissing and see straining when pooping. Give the glider a little bit of canned pumpkin (not pumpkin pie mix). If this doesnt help, a vet visit is in order as gliders can self-mutilate if they have a body part that is causing pain.
If they have diarrhea, wait 24 hours from start of problem. You can give them a piece or two of real oatmeal (not instant). If the problem persists longer than 24 hours, a vet visit is in order as the glider can quickly dehydrate.