@peterjohn,
Yep, pain in the neck.
BUT, what if she was just "checking you out?" You never "checked out" some girl you thought was cute by looking at her out of the corner of your eye so she would not notice you staring? So of what use is it to you to know whether she was checking you out or not? She probably forgot all about you 3 seconds after she looked.
As an aside, I get a kick out of the guys and gals who come here and create threads about someone checking them out simply because they believe they saw someone looking at them or made eye contact with them or they claim they saw this person staring at them. I truly believe people who make these claims must think they are God's gift to the opposite sex and are wanted and desired by anyone who happens to look their way.