Well first of all, 252 is the absolute maximum for a stat, so it is impossible to go over it.
And second of all, you can actually keep track of EVs easily by subtracting the amount of PP used when battling (as long as the move is strong enough to OHKO), or by using a simple tally mark.
You just have to remember to do so.
And if you are maxing a stat, using a Power item, with Pokerus, you can even count it in your head at 10/12 EVs each kill! (25/26 kills).
But basically, use the tally marks!
(And don't forget to use Vitamins to save time!)
Have fun EV training! :D ~