I am traveling Madrid to Istanbul on Spanair, then making my way through Turkey by land into Aleppo, Syria. I HAVE a valid Syrian visa that I obtained here at the consulate. Is that sufficient proof of onward travel for boarding in Madrid?

I called Spanair and they said they don't need any such proof, just my passport. But I'm not sure if the agent is knowledable about these things or not....

so Question: anyone travel from Barajas to Istanbul on a one way ticket? If so were you requested to show an onward air ticket? Is a valid visa to Syria acceptable proof?