Not all Pokémon are made equal—one Magikarp may kick way more butt than other Magikarp, even if they're the exact same level. That's because every Pokémon has hidden stats called "IVs" attached to things like their "attack" and "speed"—and there's a way to tell what these values are in-game.
Pokémon X & Y have been out for months now, so it's possible you've already met an ace trainer in the Kiloude City Pokemon center—he's the "IV judge." Once you beat the Elite Four, he can check your Pokemon out and tell you about their IVs.
Using this guide by Reddit user MistaWhiska, you'll be able to tell what the IV judge is saying to you, as far as numbers go—which means you'll be able to tell if the Pokemon you're raising is a clunker or not. Note that "31" is the highest possible IV value for a stat.
This, in conjunction with breeding—which is now easier than ever—should put you well on your way to making the perfect Pokemon party.
(Via Reddit)