If it a girl call them sir back, if its a guy call them ma'am . I have done this it works really well, and if they try to correct you than just say not from what I see, or could of confused me, or something else you might not say which is offensive, you have to make them aware they did something wrong, and if they argue well...management!!!. I have also very quickly snapped back at someone "not a sir" to which they apologize quickly, very quickly. You are a woman no matter what and don't let anyone tell you otherwise, even if you doubt yourself don't let others doubt you, if you come at them with conviction they have nothing....
