Whenever a Pokemon is first registered upon initial capture, the registered screen shows poke balls in the background indicating if the Pokemon can evolve. However there does not seem to be any way after that to determine if it can evolve. I have Pokemon I captured early on in my playthrough and I can't remember if they can evolve or not. Is there some indicator I'm missing in the Pokedex, or does this information become available later (I just reached the 2nd island, so perhaps I haven't unlocked that feature yet).
You should be able to see that screen once you have access to the Alola Pokedex, the lower screen lists every Pokemon encountered, while in the upper screen you have this view:
which is the same "screen with pokeballs in the background indicating if the Pokemon can evolve" you were talking about.
I've played only 3 hours on my Ultra Moon copy, and I already have that. But I've redeemed some gift Pokemon like shiny Solgaleo, so my Pokedex may be "biased".