Key things are the number of shutter actuations, the quality of the sensor and how well the shutter release still works.
Shutter actuations is an easy one, free software online works it from your last photo very easily and you should request this info from any seller. Nikons generally have a tested life of around 100,000 shutter actuations, if it's getting close to this it's probably best to steer clear (mine has reached 25,000 after 1 year)
The sensor is a bit tougher, find a good online store and they will probably clean it first anyway, but if buying privately you may need to factor that in as an additional cost.
Lastly it's just a general wear and tear thing, the sound of the mirror moving when taking a pic should be nice and 'clean' (I had a dodgy D50 at first, the shutter noise sounded very stuttery).
Keep an eye on
MPB Photographic (who have a nice cheap D70 available now), also
London Camera Exchange are very very good, they have loads of Nikon DSLRs in stock second-hand at the moment, if you have a store localish you could probably get one from another store sent over.