Click the Live Spell Check button on the main toolbar, press F12 on the keyboard, or turn on Options|Live Spell Check in the menu to make AceText mark misspelled words while you type. Click the button or press F12 again to stop marking misspelled words.
You need to have downloaded at least one spell check dictionary for the live spell check to work. If you have not done so yet, clicking the Live Spell Check button will offer to download one or more dictionaries. You can download dictionaries at any time through the Appearance Preferences.
When you double-click a word that is marked as misspelled, you will get a list of correctly spelled words that are similar to the word you typed. Double-click on the correct alternative in the list to replace the misspelled word. If the word you typed is correctly spelled, click the Learn button to permanently add it to the list of correctly spelled words. Or, click the Always Ignore button to accept the word as correctly spelled until you close AceText.