Anytime, you may start the spell-checking by clicking on located on the tool bar. The following dialog box pops up on the screen:

The spell-checker dialog gives you a list of suggestions for every misspelled word, of course word that could not be found in all active dictionaries. You may also perform word addition to one of your custom dictionaries, replace-all, ignore-all... Also if you wish to replace that word with a complete different one, type it in the edit box labeled as Replace with, then click on Suggest button.