I like to check that the in and out posts are indeed in and out posts. You can sometimes tell by the shape of the base-nut, but not always. It takes a trained eye to see the differences. You could always bring a spare bev and gas disconnect to help you.
Check the pressure via the relief valve in the center.
I like to lightly press down with my finger nail on the poppets themselves to be sure they are springing and seat back in the hole correctly. (Not a deal breaker, as if they are a little dirty maybe they won't seal back up.
The last thing to look at is the welds. There is one long one down the side, and a couple around the circumference at the top and base. Just look to be sure they are silver like the rest of the keg and not rusty. Make sure there isn't a significant amount of grinding on the welds in places, as it indicated maybe a quick welding repair, and grinding out. The keg may still hold pressure, but it could be a cruddy job on the inside. Lastly look inside the kegs if possible to see if the welds look good too.
Thats what I do when picking up kegs.
Or I negotiate for a cheaper price for an as is purchase where I don't "tear" down the guys kegs.