Hi Axie girl,
Firstly assess the overall condition of the axie. Does it look healthy? A general guide for a healthy size is to compare the width of the axie's head with the belly region. The axie should be plump with the midsection approximately equal to the width of the head. Female/gravid axies may have a rounder belly. This is normal.
An axie is too fat when the tummy region bulges out significantly more than the width of the head. Note that you only compare the tummy region. If you notice that the whole axie (limbs, neck etc) look swollen, that might be bloat, an illness condition.
A skinny, emaciated axie tend not to look too healthy. Bony prominences such as the hips or shoulder region might appear more obvious. The midsection would also look less wide than the head width.
You can weigh your axie using kitchen scales. Get a container (such as an ice cream tub) and fill with a known volume of tank or declorinated water. Record the total weight. Thereafter, transfer your axie into that container and take the total weight again. The difference in the two weights is the weight of your axie.
The best way to build up the body condition of your axie would be to offer a variety of nutrious food. Offer live earthworms, blackworms, bloodworms and axie pellets as a staple. You can give occasional treats of shrimp, fish, beef etc. This is a gradual process so don't overfeed your axie. Allow it to slowly put on weight healthily. I would recommend against feeding fatty foods (untrimmed meats, excessive mealworms or waxworms etc.) You want your axie to put on weight while ensuring proper nutritional balance. Fatty foods are not nutritionally optimal. At best, offer them very sparingly in very small amounts.
Cheers.