Plus it works the same way as the upgrading yourself with a better armor/jewelry. When you get a new equipment piece (except with sets those may not show all the bonuses like Crit Dmg) hover your mouse over it. If it is better than what you already wear you will see that your numbers color changed to green, if its worse - red. Similar with pet parts - list through the ones available and chose the best one based on the green/red.
