How do you check a gun registration prior to purchase?

MOST states do not have registration of firearms. If you are purchasing a firearm from a dealer, the dealer makes a record of the sale that he retains, If you are living in a state that does require registration, then that is performed at the time of sale. If you concern is that you may be buying a stolen weapon, and the purchase is NOT from a dealer, you can ask local law enforcement to check the serial number against the national database (not available to private citizens) Most will want to have the weapon in hand when doing this.