Have it examined by a gunsmith
If you're in the U.S. the short answer is, you can't.
It must be made by a cheap company, or not a real one.
www.airsoftatlanta.com that is where i bought all of my airsoft gun stuff at
Check w/ your local law enforcement.
Blue Book of Gun Values
you will need to find out if the store is still in business or contact the police if he registered it.
Long gun.
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.
Go the the gunshop or person you bought it from.
If selling the gun is banned, then buying it before the ban is no problem. If owning the gun is banned then it does not matter when you bought it.
Blue Book of Gun Values
Sometimes a gun show can be cheaper, sometimes a sale at a gun shop can be cheaper. You really need to shop around to find the best price. Be sure you are comparing the same model, and with used guns, be sure that you are comparing guns in the same condition.