There is no universal system that is open to non-law enforcement personnel to check such matters. A police officer can check, but will need to have the gun to do so.
You get the money for it (if you did it right.) *You can also spend time in Jail for selling stolen property.
Ask a police officer
You could probably be arrested for possession/trafficking of stolen goods.
Contact the police.
Check with local law enforcement.
You need a lawyer.
Any licenced FFL dealer. So long as it's not stolen and reported as such, you can sell it.
Have the police run the serial number.
Pawn shops are a legitimate business, and tightly regulated under state and Federal laws. To pawn anything, you will have to provide them with your ID. If a firearm has been reported to the police as stolen, information on that gun will be entered in the NCIC computer system. When you sell that stolen gun to the pawn shop, and they enter it into their inventory, the computer system recognizes it as a stolen firearm. You get visited by the police who arrest you for possession of stolen property.
There are two or three, but, they are only good if the sn is listed.
Go to the seller where you purchased the gun.
Depending on where you are, a local gun shop may be able to run a stolen gun request for you. You can do that in Tennessee, *IF* your local gun shop is willing to do it. Otherwise, you will probably have to check with the local Police Department.