With the AR-15 style rifles, the lower IS the receiver, which, by law, IS the firearm. The upper is "parts". Typically the lower is also the most expensive component.
If you are concerned that you may have a stolen firearm, contact local law enforcement. Most will check a firearm to see if it is listed on the NCIC system as a stolen weapon, and if so, to contact the owner. If it IS stolen, it is not yours. If you had a gun stolen, YOU would like to have it returned- right?
Go back to the store where you bought it.
If you think it's stolen, ask the police to run the serial number.
A receiver of stolen goods is also known as a fence.
Go to the local police with the serial number.
Go the the gunshop or person you bought it from.
Have the local police run the serial number.
Look on the weapon. If this is an instance of a stolen firearm, and you need the serial number for reporting, there are a couple different ways you might be able to find it. If you have a case the firearm came with (particularly with handguns), the number could be on a tag on the case. If you remember when and where you bought it, the shop you bought it from should be able to pull up those records.
Read it on the receiver <><><> If by "run", you mean check it against a database of stolen firearms, THAT database is available only to law enforcement. You can ask local police to check for you- they will want to have the gun in hand when they do that, in the event that it IS stolen.
Wait for it to get flagged when it is bought/sold or wait for someone to get caught with it by the police. The police typically check the serial number of a gun when it is found or somehow involved in a crime or traffic stop.
If you bought them at a shop that is still in business you might ask the owner to check his records. If the shop is closed, you are out of luck.
It can be recognised by a serial number so if you need to get it fixed the mechanic could report it as a stolen car and the police will impound it. Even if you didnt know it was stolen when you bought it you will not receive a refund and if the police suspect you did know t was stolen you could be prosecuted.
Depends on where you bought the gun. If from a store they should have the info. It might even be on your receipt. If you bought at pawn shop they usually include that info on the receipt. If you bought from individual and your state doesnt require registration you're out of luck.