Pimple or cold sore? How do I know?

This morning I woke up with a white head right in the edge of my upper lips, it's really tender but to me it just seems like a pimple. Needless to say are there sure fire signs that could help me distinguish if it really is a pimple or if it is a cold sore? Do cold sores get pus like pimples do?  I've never had a cold sore before and really hope that's not the case! Any tips would be great :)