EV checking and altering.
Checking;
The 'EV Checker' Women, or Effort Ribbon Women is a NPC (Non-Player Character) located in the Sunyshore city Market. She will check the leading pokemon in your party, and tell you either;
A). "Oh, your <pokemon>, It worked stupendously hard didn't it?"
Meaning that it has received 510 EV's. She will present your pokemon with an Effort Ribbon. Or she will say;
B). "Oh, your <pokemon> it needs to work a little harder."
Meaning that your pokemon has not yet received 510 EV's. Or she will say;
C). "Oh! That Effort Ribbon looks good on your <Pokemon>."
Meaning it has already received 510 EV's in total, and been check. Note, she will still say this if you wipe EV's away from a pokemon, as long as it holds an Effort Ribbon.
An Ideal method for checking your pokemon, for example a special sweeper with 6HP / 252 Sp.Atk / 252 Spd. Is to give 10 Calcium vitamins (100 EV's in Special Attack) and 10 Carbos (100 EV's in Speed), then EV train so both stats contain 252 EV's. Then show the pokemon to the EV Checker at Sunyshore. If you have done it correctly, she will say option A. Then give a Single HP up to your pokemon, and check again. If she says option B. Your pokemon has very likely been trained correctly.