A spell-check feature is available in the WYSIWYG editor, but is disabled by default.  There are two spell-check icons in the top bar of the editor window - both showing "ABC" and a checkmark.  Use the one on the left to check the entire body of your post one time.  Or to have the editor spell-check as you type, click the icon on the right, then select "Enable SCAYT" from the dropdown menu.