In this species, you can't go by the belly because they puff up when scared. Check the tail base (just below the back) It shouldn't have indentations but be fairly smooth or rounded.
Sometimes they slow down in the winter because of lower temps and you should be monitoring temp changes, raising temps with lighting changes when needed.
They also get bored with their diets and you can help by offering their favorite color orange (shaved carrot, squash, pumpkin)mixed in with greens.
Occasional pieces of fruit like cantelope, apples, pears in small amounts can give them hydration and help appetite.
And don't over dust their food with supplements. That can be a turn off.
Hope that helps!
GinaG