Make sure the revolver is mechanically sound. Ensure it locks up solid, there is no indication of severe fouling around forcing cone and cylinder, no bulge in the barrel. Remove grips and check for condition. I would say if it is in at least 80% condition appearance wise you have a winner. If the factory box and etc. are with it, then all the better.
Make sure you know who the seller is and at least get a bill of sale. If you do not know the seller, get his or her driver's license information. I do not know what the laws are where you are at and if you need to process through an FFL.
You do not want a stolen gun and also make sure there is no issue with the serial number like appearance of altering or attempting to remove.
Meet the seller in a secure public location for your safety. This is if you do not know the seller.