By default, you should have spell checking turned on in the EHR. If you'd like to disable it (or you've accidentally disabled it and would like to re-enable it) just right-click in the white-space of any encounter note and you should see a menu option for "Spell Check".

Hover over this and make sure that "Use basic spell check" has a check next to it (or uncheck it if you'd like to disable spell-checking):

Expanding the Spell Checking Dictionary

If the browser is regularly flagging words that are spelled correctly, it's every easy to add these to your dictionary. Just right-click on the underlined word and select "Add to dictionary":