All animals can over eat. Even tortoises.
My vet showed me a quick way to determine body condition under the shell. (You can also gently press the visible tissue between tail and leg, but you can hurt a tort pushing on their abdomin so leave that to experienced handlers/vets). The ridge down the middle of your tortoise' shell follows his spine. A malnourished tort will have prominent ridge and have a sunk in in appearance. A fat tortoise will have very little visible ridge and sometimes the fat under the shell will cause a little fat bump on either side of the spine ridge. A healthy amount of fat is somewhere in the middle, you can still see the spinal ridge, but shell is still nicely rounded . Now this is not 100% foolproof, but it is a way you can gage "is my tort fat?". Note this worked better with older tortoises than younger ones.