If you have a lot of rare candies then you can use Serebii's IV calculator: http://www.serebii.net/games/iv-calcdp.shtml
Save your game then level up to level 50 with rare candies (so you don't have any EVs). You can do it by battling but you need to know the exact number of EVs you have which you don't always know.
You can then plug in the pokemon's current stats to calculate the IV. The closer you are to level 50, the more accurate the answer will be. It will give you the possible range; if you are only lv 10 it will be something like "10-30" which isn't helpful.
The other way to get a rough guide is to ask the man at the Battle Tower. He will say the "Pokemon's potential" which tells you the total of all the IVs:
Decent = 0-90
Above Average = 91-120
Relatively Superior = 121-150
Outstanding = 151-186
The same guy will also say what stat has the highest IV, for example if he says your attack is fantastic, attack is your highest IV. You can also check your Pokemon's characteristic as described here: http://pokemondb.net/mechanics/hidden
