The Registro de la Propiedad (Land Registry) is an institution where all the properties of Spain have an ID number and any citizen can look up (having that ID number) for the legal situation of that property (who is the owner, burdens such as mortgages, seizure for debts of the owner, etc ).

It 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 of the house, if it is already built) and we check that those details are on the private contract and next we look for the information in the Registro de la Propiedad and within a few days we know who the property belongs to and what are the burdens, if any.

In Spain, all the burdens that are related to the real estate (like mortgages) must be registered in the Registro de la Propiedad and if they are not there they do not exist, so a mortgage to be effective must be registered. Looking up the Registro we see wich burdens are registered on it or if the property is free of them.