The most EVs a Pokemon can get in one stat is 252. Is there a way to tell when to stop training in one stat?
What I do when I have maxed out a stat I keep one of all Six stat training bags.
Click on the bag of the stat you have maxed out hit use and if your stat is 252 it says that your xxx stat is already maxed
give it a bag of the corresponding ev if it says his base stat cant go any higher that means it is at max.
Also when a Pokemon has gotten all the evs it can have a buff figure will appear in the top left cornor of the screen meaning they have max evs.
Hope I helped