You can politely call the local police (non-emergency line is probably best) and ask if they are willing to run the serial number. I don't know if anyone has actually done it or not. Aside from that, there is no way for you to verify if the pistol is stolen or not yourself.
I have bought used pistols and I don't bother calling the police or any of that. I just trust my gut. If the person is acting shady, then I will not buy from them. On the other hand if they are knowedgable about the gun, have a plausible story where it came from and are willing to show me their CPL and DL (I write a reciept for them to sign that I keep that includes their name, address, gun model and serial) then I will do business with them. I highly doubt a criminal is going to be that open, so I feel safe enough. If you purchase a stolen weapon then you are the one in possession of stolen property (also you will be out your money too), do what you need to to be comfortable or walk away from the deal.