Spell some words wrong and see if it corrects them. But attack it systematically. Use spaces, wild cards, symbols and other non-supported characters. Then, pull the database for the spell checker and validate that everything in it is accurate. This will be done manually the first time. Then establish that db as a baseline to test against going forward. Add new words, etc. and validate. There are hundreds of tests to validate a spell checker, just think about the possibilities!
If this is the same question as at Yahoo Groups, and if you want to USE WinRunner to PERFORM a spell check, you already have your answer. You can't do it. (Well, I suppose you COULD, if you wanted to build an entire spell-checker in TSL, complete with a dictionary).