The spelling checker underlines unrecognized words in red. You can select alternatives from the context menu.
Control–click in the Letters text box and a context menu appears.
In the context menu choose Spelling and Grammar > Check Spelling While Typing.
Enter the words "Roses are red, vilets are blue" in the Letters text box. The word "vilets" is underlined in red.
Control–click to select the word. A context menu appears, with suggested alternatives listed at the top.
Choose the word "violets" from the context menu. The word is replaced in the Letters text box.
You can also use Learn Spelling to add words to the dictionary, for example your name, or use Ignore Spelling to ignore the suggestion.
Alternatively, choose Correct Spelling Automatically to see suggested replacements for unrecognized words.