If you need to communicate with a subject who claims that he can't speak a lick of English--but you suspect he can--here's a quick trick that can help you confirm or eliminate your suspicions.
Begin asking a fast-paced stream of the usual questions like, "What's your name?" "Where do you live?" "Is this your car?" "Do you have a driver's license?" Of course the response you'll get is "I don't speak English" or "I don't understand" spoken in his native language.
In the middle of your fruitless questioning, suddenly drop a comment--seemingly to yourself but clearly loud enough for him to hear it--like, "Man, I knew it! Guys who don't speak English always wear that kind of belt"…or always have those kinds of buttons on their shirt, or always wear those types of shoes.
Odds are pretty good that if the subject does in fact speak some English, he'll reflexively look down to the area you refer to. With that, you've got your answer.