The Registro de la Propiedad (Properties Register) is an institution where all the properties of Spain have an ID number and any citizen can apply (if you have that ID number) for the legal situation of that property (who owns it, what charges has on it like mortgages, seizure for debts of the owner).

Is very easy to check if the builder is the real owner of the property. We would just have to require the builder for the legal details of the plot (or the house if this is already built) and we check those details are on the private contract and we apply for the information to the Registro de la Propiedad and within a few days we know who the property belongs to and the charges it has on it, if any.

In Spain, all the charges that are related to real estates (like mortgages) have to be inscribed in the Registro de la Propiedad and if they are not inscribed they do not exist, so a mortgage to be effective has to be inscribed. Applying at the Registro we see if any charge is inscribed or if the property is free of any.