Spell check will make misspelled words with a red underline while typing.
Setting path:
realme UI 4.0 & above versions: Settings > System & updates/Additional Settings > Keyboard & Input method > Manage keyboards > Gboard > Text Correction > Spell check
realme UI 3.0: Settings > System Settings > Keyboard & Input method > Gboard > Text Correction > Spell check
realme UI 2.0 & 1.0: Settings > Additional Settings > Keyboard & Input method > Gboard > Text Correction > Spell check
realme UI R & Go & S & T Edition: Settings > System > Languages & Input > Spell checker