In Pokemon Sun and Moon, it is impossible to tell exactly what EVs your Pokemon has, at least not down to a specific number. However, since Gen 6, it has been possible in one way or another to access a graph that visually displays your Pokemon's EVs. In Sun and Moon, you can do this by pressing Y on any Pokemon's summary screen. Doing so should bring up a hexagonal graph that looks like this:
The blue layer outside the yellow one at the core of the graph indicates where the Pokemon's EVs have been distributed. Any stat that has reached the full capacity of EVs at 252 will sparkle, as Speed does in the image above, and if the Pokemon has not reached the total capacity of EVs at 512, the graph's blue layer will be orange instead. For clarity, the yellow layer represents the Pokemon's stats not including EVs, and becomes more exaggerated as the Pokemon gains levels.
Whilst this is the best way you can determine EVs, it's not the way you should go about adding EVs to movesets in Pokemon. EVs are generally accepted only as a number, not a graph like this, so unless the stat is maxed as indicated by the blue outer layer, you aren't going to be able to provide a specific description of your Pokemon's EVs. Also, as mentioned in the comments, EVs are planned in advance and designed to be optimal for your Pokemon, and any moveset that makes use of EVs randomly gained from battle is inherently flawed. Generally, the EVs your Pokemon has will only make for a good moveset if you planned them in advance; if you haven't carefully selected what battles the Pokemon participates in until it maxes its EVs, then your Pokemon aren't optimised.
If you'd like to clear your Pokemon's EVs, head to Route 10 and pick up the berries by the large trees. The berries you'll get will raise happiness and decrease 10 EVs in the stat it corresponds to. Farm them in Poke Pelago and you should have enough to decrease your Pokemon's EVs to 0 and then start over. There are plenty of EV training guides online, so use them to your advantage here.
