There is a simple way to tell if you are feeding your axolotl adequately. A healthy axolotl has a belly about as fat as it's head. If your axolotl does not have this, it could do with some extra feeding. However, a male tends to be a bit more slender and stream-lined than a female, so won't always get so fat, so if you have a male and female, don't try and get them looking identical in fatness.

 

The picture on the left shows two healthy females on the left, and two males on the right. The males bellies are about the same width as the head, they just aren't as round as the females.