Oh, I misunderstood, I didn't realize in this scenario you were asking them if they were nonbinary. The linguistic answer is everything in Spanish defaults to masculine.

I, personally, would treat it the same as I treat the pronoun game here in the US, because it's essentially the same thing: I start with whichever one jumps out at me and accept correction if necessary, because they are the ones who made the decision to make their grammatical identifiers differ from convention. It's not my responsibility to know it ahead of time.

If they want to be a dick about it, I now know they're not someone I want to spend time around anyway.