It can depend on how your database is set up. I assume you require an answer for each question, so you can check to see if there is a value for the questions you asked, or you can create a new field that is set whenever information is written to the database for a certain user. Generically, the easiest way to check to see if someone has taken a poll is to check if their record exists (assuming you are generating a record when they take the poll.)

Brian