There is no specific procedure in Spain to obtain reliable information on what are the existing assets of a deceased person. You should try to narrow down your search a bit more, for example, whether you are looking for real estate, bank accounts or other goods.
Also, find out the area in which your relatives lived, because Barcelona includes the city and the province, and it is the second biggest in population in Spain. Once you know the area, for a start you should identify the people who can help you: locals who would have had any contact, friends, acquaintances, or neighbours. Focus on those places where the elderly tend to congregate. A visit to the Town Hall of the municipality is a must, as they might have interesting information, such as the last domicile of your relative.
If you obtain information, you could approach Land Registries and banks in order to do a proper search. If you do find assets in the name of your relative, you can then file a procedure to transfer ownership to the person entitled to it, your father or yourself.
