The spellcheck is on by by default but if it's off and you’re in a script, touch the left-pointing Back arrow at the upper left):
Touch the App Settings icon at the upper left...
...and switch the spellcheck on:
Touch the screen to exit this window and touch a script to open it.
If a word is misspelled, it will have a dotted red line under it ("precocius").
Touch the word and a menu with a spelling choice or choices will appear:
Touch the correctly-spelled word or choose to Ignore, Ignore All or Learn; the word will be corrected and the spellcheck bar will disappear.