I live near the Mexican border in the city of Chula Vista (near San Diego). I encounter Spanish speakers right and left. Some don’t speak English but I never ask them if they do. I simply say I speak English.
I usually speak in English and if they don’t speak English they will tell me. Or if it obvious that everyone is speaking Spanish I’ll ask them “hablas engles’” and they know I’m just looking for someone I can talk to. I think these days since it is not PC to speak only English, it could be that people that only speak English don’t know how to go about finding an English speaker in a PC manner.