Write down the spec of every part fitted.
Any dents, scratches or marks to the frame that you can recall? If so when and how were they caused? Write those down too.
The police should record that in a statement from you and then go and look at the bike to check those points and interview the current “owner” about how he came by the bike. Ultimately it isn’t always cut and dried and the cops may refer it to a civil interpleader claim – especially if it has changed hands several times or been bought from a shop/semi legit process but if the buyer has any doubts at all about the legitimacy of his purchase they can usually be persuaded to relinquish any claim.
Good luck