Checking if user has drawn a circle?

Hi,

I'm working on a little something where users can draw their own shapes and flash will identify those shapes. For example, if a user drew a shape similar to a rectangle flash will fix that shape into a perfect rectangle.

But I've hit a problem, I can't figure out how to check whether a user drew a circle or not.

Does anyone happen to have any idea about how to do that?
I'm working with AS3, but any version of AS would apply. Heck, doesn't even need to be in AS - Just an idea will do!