There are 2 useful ways to tell the amount of IVs your Pokemon has..
If you're looking to see exactly what IVs your Pokemon has, then you could use a stat calculator such as this one from marriland. (This calculator will work for generations III-VI) However, you will need to know what EVs your Pokemon has in each stat, which is kind of a bummer.
If you're willing to use EV reducing berries to reset your Pokemon's EVs, there is a Pokemon Breeder♀ next to the west entrance of Nimbasa City who will sell 5 of the same type of EV reducing berries to you every day in Pokemon White 2 and Black 2. The type of EV reducing berry she gives you depends on your leading Pokemon's highest base stat. You can also receive every type of EV reducing berry from a rank 10 Join Avenue florist.
In Pokemon X and Y, you can use a Reset Bag in the Pokemon Training Regimen Screen to reset your Pokemon's EVs. You can obtain this bag by having your Pokemon punch the default, clear punching bag while you still have an empty slot in your punching bag inventory.
If you're just checking to see whether or not your Pokemon has perfect IVs or if you're alright with just a rough explanation of your Pokemon's IVs, then you can consult the IV judge, who can be located in the Gear Station in Nimbase City in all Generation V games or in the Pokemon Center in Kiloude City in Pokemon X & Y.
The amount of IVs your Pokemon has in a certain stat will depend on what the judge says:
- rather decent: 0-15 IVs
- good: 16-25 IVs
- fantastic: 26-30 IVs
- can't be better/can't be beat: 31 IVs
Bulbapedia Article on the Stats Judge
Join Avenue Florist Merchandise Chart from Pkmn.net