well, between just coming out of pouch and about 12 days, their eyes are closed and their tails are kind of pinkish without much fur. Their heads are kinda too big for their bodies and they are peeping (crying) a lot.
Once their eyes open, their tails start to "fluff out" a little and they start acting a little less wobbly.
Their bodies begin to fill out more in proportion to what an adult looks like between 6 and 8 weeks.
They will continue to nurse up to 15 weeks, so if they are continually sticking their heads in mom's pouch, they are not yet 15 weeks. She won't allow it after that time. This according to the Merck Veterinary Manual and our own observations.
But if they are not with mom, there is no way for them to continue to nurse if they are less than 15 weeks. This would be the case if you just bought your glider from a mill breeder or a pet store or trade show front-man for a mill breeder. If this is how you got your glider, it is not likely the glider is any older than 8 weeks. Mill breeders yank them off of mom only half-way through the natural weaning period to keep the baby machine running. Sick scum that they are. This can also be the case for a hobby or home breeder that just doesn't know any better. A lot of non-mill breeders seem to think 8 to 10 weeks is ok, but that is not the natural weaning period as much as they don't like to hear the truth. That weaning period is necessary for dad to teach them how to eat solids, how to play, how to pot, how not to dirty the nest, etc. If you yank them sooner, they are the crabby scared-to-death mess that everyone experiences with too-young joeys.