Get a counter, or see if theres one online, and just add 1 every time you add an EV.
This is why EV training was so easy in D/P/Pt, the bottom screen had a counter app to easily keep track of your progress.
That's really not necessary. Super Train shows how many EVs your getting, and you can always look at your Pokemon's actual stats and the EVs will already have been applied, it's a moot point.