The best way to check your Pokemon's IVs is to enter a battle in the Battle Subway or the Battle Institute with 3 of them. Their levels will be auto-increased to level 50 for the duration of the challenge (the higher a Pokemon's level, the more easily its IVs can be determined). However, it's a better idea to do the Battle Institute, since the Battle Subway forces you to save before the challenge, considering that you might not get the IVs you want and you don't want to spend a lot of time releasing Pokemon you don't want.
But, go to the battle facility in Nimbasa City, save before entering, and then enter the tests with a double battle style. The game will level your pokemon to level 50, and then you can use an IV calculator, (google it, Serebii's works best for me) and voilá.
This method only works 4 pokemon at a time, that's why I recommended that you save before entering. Once you write down your pokemon stats, or check them online, just reset your game, and enter with different pokemon.
Pearl : 2538 0185 7851
SoulSilver : 2495 0636 9069
White : 0905 9140 0338