Your search appears as a lengthy and challenging task. Different steps you should follow are:
- Draw up a list of possible assets he left in Spain (real estate, cars, bank accounts...)
- Narrow the search to a particular area where he was living or where you think he may have left assets.
- Do a search into the different land registries of the area.
- Contact people in the vicinity to find out more.
- Apply for a death certificate in the location where he died.
- Apply to the central registry of wills for a copy of the last Spanish will (an "open will" signed before a Notary Public, since these wills are all registered in this administrative office).
- If there is no available open will, try to look for a private one if you think he might have left one.
If he is a US citizen, get a copy of the will and see what are the dispositions regarding overseas properties. If you cannot find a will, without being good news you can still proceed: You could file an intestacy procedure in a Spanish Court in order to have the assets transferred to the legitimate heirs. Obviously, assuming you can find assets.
