Go over
here and check the column labeled "Melee Dmg". That's the minimum and maximum a pet a can hit for.
All pets of equal level hit for the same maximum amount (e.g. all Elemental: Earth summoned pets that are level 13 will hit for 18 max), and the inverse is also true. Hence when you see the max hit of a pet you can know what level it is.
That being said, and as already mentioned above, it might not be in your best interest to continually summon/reclaim pets to get a max pet every single time.