What is a car-rental agency responsibility in checking for an appropriate license?

The rental agency cannot be held liable or "at blame," an international driving permit is required and will be asked for to rent a car in Spain. For example, here is what Europcar says: http://www.europcar.com/EBE/module/render/faq

In addition, the below is taken from the U.S. Embassy site for Spain: (which can be found here: http://madrid.usembassy.gov/citizen-services/faq-spain.html)

Q: Can I drive with my U.S. driver’s license in Spain?

A: U.S. citizens visiting Spain who want to drive in Spain must obtain an international driving permit prior to their arrival in Spain. An international driving permit (IDP) translates your state-issued driver’s license into 10 languages so you can show it to officials in foreign countries to help them interpret your driver’s license. The IDP is not valid by itself and must be carried with your driver’s license. More information on driving overseas.

U.S. citizens who are residents of Spain must obtain a valid Spanish driver’s license. At this time, there is no agreement between the United States and Spain for the validation of a U.S. driver’s license; therefore, holders of a U.S. license must attend a Spanish driving school and take the Spanish exam. You can find more information on the Spanish Minister of the Interior’s website.

In all cases I know of you are always responsible.

"It is your responsibility to know the laws in effect where are you are." I am not sure about US where there are lots of weird stuff going on, but I doubt it is any different there. Pleading ignorance and blaming others for not educating you usually doesn't work in the face of the law ;)