The easiest way for most people is to use Wi-Fi Battle with someone and set the battle to LV. 100 All. Then, enter the stats into an IV calculator (I usually use
Serebii's IV calculator) and the results will show you what the IVs of your pokemon are. You have to know the EVs of the pokemon or the results will be wrong. This only works if you have Wi-Fi to do the LV. 100 battle.
And as Wulf said, you can request an IV Battle in
this thread.