If you can, take a laptop with card reader and a white target to shoot. Take a picture of the target, and a picture with the lens cap on, and look at each color channel in Photoshop to see how many pixels are stuck on or off. This may or may not matter to you. There will always be some, normally if they aren't clustered together they aren't that noticeable.
Mainly take some test shots and make sure everything seems to be working properly, remove the lens, check inside the body for dirt or fingerprints (fingerprints shouldn't be that harmful, but they do indicate that the person didn't take the best care of their gear). If it's working when you get it, that's the best you can hope for with used gear. As others have said, try to figure out how many shots they have taken vs the expected # before shutter replacement. Nikon I think encodes this in each picture, most (all?) canon cameras are much more difficult to check. If they are numbering the pictures sequentially, that could give you some idea, but my body rolls over every 10000 shots and shutter failure is supposed to be at 50K or so, so unless the person has been keeping track of how many times it has rolled, you won't really know. I suspect that most (but not all) digital cameras at the bottom end of the price spectrum will have many many shots left in them (lots of people buy thinking they want a DSLR then change their minds). At the top end, the cameras typically get used more, but should also be good for more shots before failure.