All you need to do is cut and paste the references from the end of your paper into the textbox in the format checker. Then click Check. (Note: make sure you don't have any blank lines) It will find common errors in formatting such as leaving out an author's initials or the final full-stop on a line. Note that it does not check whether the references themselves are correct, although that would be possible with current APIs.
When your eyes are hurting from looking at one too many "et al."s, try the format checker - it's a cite for sore eyes.