it's not a reliable method with gliders really. They hide most illnesses very well. Usually by the time you see symptoms they have been sick for sometime. Still it's good your observing him and getting used to what his normal is so you'll notice when it's NOT normal
Over grooming/bald spots that arent the scent gland, wet, and cracked fur, are what we look for in gliders fur in regards to something being off.
bald spots on the tail can be a sign of a broken tail, and sometimes stress, wet fur can be a sing of bacterial infections, tooth problems, fungal infection so many different things.
If your unsure if he's healthy, take him in for a wellness exam they should weigh him and look him all over, fecals are always a good idea to rule out parasites...