Have you received a printed GC bill?
Test your bill's password (it might be written on the back) and see if the resulting "hash" matches the target code listed below for your bill.
Test password here.
Whoever enters the (correct) password into the blockchain owns the bill next - and can either set the printed bill's next password, or keep the bill online-only. See the Whitepaper for complete instructions.
You can also confirm all aspects of the bill's validity by searching for its serial number in the Stellar Lumens blockchain - details in the Whitepaper.