I think you guys are exaggerating the inflation of price on this one - $675 is not bad, and I can tell you from my local craigslist, there aren't any other 13" MBPs in that range. They're all $700+. There are no completed ebay auctions in the $400-$450 range for this model, unless there's damage of some kind.
Refurbs are the best bet, of course, but that's expecting the OP to pay almost $400 more after taxes. I would maybe offer $600 or even $650, but it's not a crazy price.
So to answer the original question:
1) Click the apple in the upper left menu, and select "about this mac" and "more info..." - this will give you the detailed specs of the machine.
2) Test all the keys out, making sure to be aware of any stickiness or crunchiness when a key is pressed, which can be a sign of liquid damage.
3) Bring a DVD to test the superdrive.
4) Make sure you meet somewhere with wireless, so you can test the airport, check the serial, and make sure that's working properly.
5) Finally, you can run the Apple Hardware Test using the OS X install disc that comes with the computer:
http://support.apple.com/kb/HT1509
Good luck.