It is easy to judge the character of your broad by this action. If she accepts the anal attack, you can bet your broad gets around (or just really loves it in the ass), yet if she denies, she's a prude. Not to stereotype, or anything. You're looking for an immediate rejection, followed promptly by acceptance.
"She made this noise...like....a....a..."
(Jammin' in the ass-cannon)..."SqueeeeeeeEEEEEEEE!"