An overweight Corn may:
a) Look like its head is too small for its body - kinda out of proportion
b) Look like it's wearing a sweater two sizes too small - you'll be able to see skin between its scales
c) Have a completely round cross-section
d) Have "love handles" - be unusually flabby in the lower end, which is particularly noticable just before the vent.
e) Feel "flabby" or too soft when picked up - lack muscle tone
If your Corn is about the right size, you shouldn't see the first four. Some Corns are naturally relaxed when handled and each has their own individual muscle tone, so that's more something you might notice if you knew the snake well to start with. There will be a bit of definition around the spine, which should be visible along the back, but it shouldn't be prominent.
An underweight Corn will have a spine that really sticks out. You may see looser folds of skin where it has lost weight and the skin has remained the same size. It might feel light and weak when picked up. It could be dehydrated - look for sunken eyes.
If your Corn is a normal size to start with and you're unsure which way to go with feeding, my advice would be to err on the side of slightly underfeeding. Putting weight on a Corn is all too easy, but getting it off again is a lengthy battle (I've been there!). Underfeeding is much easier to remedy.
