On iOS 8, how do I turn on spell check without turning on Auto Correct?
Settings ➙ General ➙ Keyboard:

Turn off Auto-Correction

Make sure Check Spelling (at the bottom of that same screen) is on

Your iDevice will no longer automatically correct misspelled words, but it will highlight them for you. As far as I know this is the only way to have spell-checking on without the Auto-Correct feature.