- Highlight what you wrote and Control-click on the highlighted text.
- Select Spelling from the contextual menu and then Check Spelling from the submenu.
[robg adds: I find it highly ironic that this hint was submitted as "Youc an spell check..." :-). In any event, this is not really a Safari feature, it's a Cocoa feature, and it's available in any Cocoa application with an input box. It would be nice, though, if (like iChat) you could permanently enable "Check Spelling as you Type." There's a menu item for it (Edit -> Spelling), but you have to select it for each input box, every time!]

