How do I check what kind of power supply I have in my computer?

I’m thinking of getting a new video card that has a requirement for a 450w power supply. But I can’t seem to find out what power supply I currently have. I don’t see it in my hardware profile, and even when I open the case the power supply doesn’t have any distinguishing marks on it that I can see (at least, without dismantling the whole rig.) Any ideas on how I can find out this information?