All Pokemon have IVs. What I think you mean is, how do you know if they have perfect IVs or not. In which case, there's a couple of things you can do:
In-game, there's an IV checker (like in the Battle Resort Pokecenter in OR/AS), who will tell you that certain stats of your Pokemon are excellent or very poor and so on. "Cannot be better" for example is 30-31 IVs, i.e. perfect IVs. "Can't be worse" or "Dismal" (along those lines, I don't recall the exact things) is 0 IVs. Here's the link on Bulbapedia about this.
You can use IV calculators online. By inputting the stats of your Pokemon with all relevant information like EVs invested and nature, the program gives the mostly likely IVs your Pokemon has. A popular one was Psypokes, but I don't think they've updated in a bit.
