One thing you could do is ask the person selling the car how often he cleans the drain on the driver's side of the car. If he can't give you an answer (or says the dealer has done it as part of maintenance then you can't be sure. If the seller does give you an answer, ask him to show you where the plugs are. If he can't, then you've got a less than credible seller.
[edit]And if he can show you where the plugs are, gets on his back, reaches up and pulls out the plugs and shows you that they are clean then you have more confidence in the seller.
Here's one thread on this topic:
http://www.zpost.com/forums/showthre...ighlight=drain