Buying cornelius kegs second hand, what to look for?

My wife bought me a bunch of equipment to build a keggerator for my birthday (yay!), but no kegs since she had to have the stuff shipped from the states and that would have cost an arm and a leg. I found a classified selling used cornies locally, but since I have zero experience with them, I could really use some tips about what to look for when I check them out; from the ad it seems like the guy doesn't know too much about them either.

So, a couple specific questions:
1) how can I tell if they're pin or ball lock? (doesn't really matter to me, but I'll need to get the right disconnects)
2) what should I check to know if they're in good/serviceable condition? I don't mind replacing gaskets, but I'd prefer to not have to replace dip tubes and lids and such (though his price is not bad, so things that are cheap to fix are ok).
3) Anything else I should know.

Thanks so much!

I usually just care that they hold pressure and the inside appears clean. Seals and poppets can be replaced later if needed.

Pin locks have both disconnects on the same side typically and are 3" shorter than ball lock.
Example - http://cornykeg.com/catalog.asp?prodid=668637&showprevnext=1

Ball locks typically have the disconnects across from each other.
Example - http://stores.kegconnection.com/Detail.bok?no=1

Some people convert pin lock to ball lock, but the dead giveaway between them are the two small pins on each disconnect.

they are essentially ALL second hand. Most of them are 25 years old.

Watch for dents and dings around the lid opening.

To check pressure, ask if your seller can pressurize them with some CO2. Or do it yourself if you have a tank and regulator already. You shouldn't see/hear any leaks and can relieve pressure by pulling the relief valve or pressing down on the gas poppet.

I'm sure they'll be fine though, as long as there are no dents near the lid, as DeafSmith mentioned. Don't worry about dents on the body. As long as it holds pressure it'll be fine.