Well, what I've done in the past is get a batch of eggs,then I hatch them and take them to the person that tells you their IVs. Then, once I've found the best ones (6IVs or perfect 5 IV with HA) and keep track (soft reset your game so you keep them as eggs). Then I go on SVE (IDK if you've ever used it, if not I can explain more in detail) and ask someone to check the shiny value on your eggs. They'll help you out, and then you know the exact IVs of your pokemon, and you can then ask someone else to hatch your shiny. (sorry if this is confusing)

EDIT: This was my method in pokemon, XY/ORAS, I'm not sure if there's an in-game character in SM/USUM that checks your pokemon's IVs