Fairly generally, if you right click on a blank spot in a toolbar you will get a menu of things you can do to change the toolbar. This works on lots of different Windows bars and for quite a few applications like browsers.
On the Google toolbar, the menu includes only the items you don't already have icons showing for, but if spell check isn't up, it should be on the list. (The label is just "Check" with a little green checkmark on an "A" for the icon picture.)
Mine disappeared a while back, probably because of either an IE update or a Google update. Never figured out where it went, but I finally managed to get it put back with the right-click trick.