How can I check a used Imac if is stolen before I buy it

What model, age is the machine. If it is Lion or before, very unlikely. There are no records kept at Apple and no other resource other than possibly a local police station.

You would have to take the serial number to the police and ask then to run it on NCIC.

The only other possibility is to do an internet search by serial number. There is a remote possibility that someone who had a computer stolen posted something like, "Hey! Someone nicked my computer It's serial number is XXXX. Call me if you see it."

Make sure the seller has prepared the iMac for sale according to this Apple document: What to do before selling or giving away your Mac - Apple Support. If it's not prepared that way there's a good chance that use of the App Store and other features requiring your Apple ID will be blocked.

When it's booted up it should present the setup window as if it were a new iMac.

If it's old enough that I would have come with Snow Leopard or earlier as the operating system make sure the disks that came with the iMac are included.

I would not purchase one if it hadn't been prepared that way or unless I knew and trusted the seller so that if there are problems that pop up they can unlock them.