How do you test for oil base verses water base paint?

I just asked this question at a local paint store and tried it at home. You put some rubbing alcohol on a cotton ball and rub it on the painted surface (you might want to do this in an inconspicuous place since it may leave a mark on your wall). If some of the paint comes off it is probably latex (water-based) paint. If it does not it is probably oil-based.