Final Draft comes with powerful language tools: A 120,000-word spellchecker and a thesaurus, both powered by Merriam-Webster. As in other word processing software you've used, a right-click or command-click on a flagged word gives you spelling choices; another mouse click replaces the word and of course you can choose a full sweep of your script any time you want.
Names, places, sound effects and words you've created can easily be added to your customizable user dictionary.