Ampys dont lose/gain anything post evolution if you used them before evolving. They are a flat gain only dependent on the amount given. Do note that you can only give 100 of each ampy (HP/ATK/DEF) with a maximum stat of 3000 HP 1000 ATK 400 DEF.

I don't think there is a way to determine how many of each you have added except by doing division. This can be checked on the character details.
The blue indicates ampies were fed on the stat


The yellow indicates bonus stats from ampies - in this case 4 HP 3 ATK 3 DEF