It can be done after the theft. The shop writes the serial number of the bike (stamped on the frame) on the receipt. If the bike is stolen, the owner reports

It can be done after the theft. The shop writes the serial number of the bike (stamped on the frame) on the receipt. If the bike is stolen, the owner reports the number to the police. The police can check any bike in their database of stolen bikes [1]. People purchasing second hand bikes will be wary if the seller doesn't have the original receipt. They can also check the frame numbers in the police database [1]. [1] https://politi.dk/cykler-og-koeretoejer/tjek-om-en-cykel-eller-et-koeretoej-er-efterlyst/tjek-om-en-cykel-er-efterlyst https://politi.dk/cykler-og-koeretoejer/tjek-om-en-cykel-ell...