How do I know what EVs my Pokemon has, or when their EVs are maxed out?
Unfortunately, there’s no way to know what EVs your Pokemon has unless you have been keeping track of each and every Pokemon you battle.
However, in R/S/E, there is a woman in the Slateport Market who will give your Pokemon an Effort Ribbon when their EVs are maxed out (all 510). There is no way to know if your Pokemon has maxed EVs in FR/LG.