What I did: Looked for a way to turn on spell check
What happened: Couldn’t find it
What I expected: Spell check to kick in automatically when I started to write a note
Things that might be helpful to know (Agenda version, OS and model, etc): iOS
On iOS, spelling should work the same as in all apps, I believe. It probably depends on settings you have in the Settings app. You should have the normal autocomplete to help with spelling too.