Well, I guess you can compare it from what you started and where you ended. The next thing you can do is to check the yellow line under your pet's picture after a battle and get a certain idea how much XP it got.
I haven't ound a place where the pets XP is calculated, but the guideline "lower leadership than the enemy + hit/kill more opponents" usually works for me. Come to think of it, I rarely look at my pet's XP, simply use it as I see gfit and select the level ups
