The easiest way to tell is by first plugging it into an IV calculator like Psypoke's, and/or use the stats Judge in the Gear Station.
Sometimes if your Pokemon aren't leveled high enough, it is hard to tell what their exact IVs are. Using a combination of the two may solve your problem.
Another method you could use is by saving and then entering the Pokemon you want to check in the Battle Subway so when you enter it sets it's level at level 50. Once the battle begins, check your Pokemon's new stats and it should give you accurate results using the IV calculator and stats judge.
However, if you used your Pokemon in battle before and it gained EXP, it might have some EVs that affect its stats. In order to find out how many EVs your Pokemon has, you have to know every single Pokemon it has battled against. If you don't know, it can be hard to tell. I shouldn't recommend hacking tools, but hacking is one of the few ways that can tell you how many EVs the Pokemon has. If your Pokemon has never been in battle before, then ignore this entire paragraph.
Good luck. Ask if you have any more questions. :)