I have been using this tool for some time now. It’s simple and straight forward. It will tell you if your colour combinations are good or not. Large and bolded text are categorized differently than smaller text. The program also tells you what priority your combination represents. Priority 3 (AAA) is the lowest level, so for all intents and purposes, this is the minimum requirements. Although the page is a little clutered for my liking, I bare it. After all it’s free!
Check it out at: http://www.colorsontheweb.com/colorcontrast.asp