Probably not online, but any local police station can run the serial number and tell you. If it did turn up stolen, you wouldn't be in any trouble, but you would lose the gun.
Go to the seller where you purchased the gun.
Contact your local law enforcement. They can check to see if the serial number has been entered as a stolen item. IF there is a report, the officer will need to retain the gun to return it to the owner. They WILL need to have the gun in hand to do this, not over the phone.
Ask a police officer
Contact the police.
Check with local law enforcement.
Expect the police to expect YOU to explain how you came to possess a gun that had been stolen- just like any other property.
You need a lawyer.
Go to your local police with the sn. If it's been reported as stolen they can tell you.
Contact the police
You can find a bubble gun online in online stores such as Amazon or the online store eBay. In addition, you can find a bubble gun in a toy store such as Toys R Us.
There are two or three, but, they are only good if the sn is listed.
Contact local law enforcement. Give them the gun, and ask if they will check the NCIC data base (as a private citizen, you do not have access to that system). If the gun IS stolen, it will be confiscated by the police, and you may have to testify in a stolen property trial.