I've never been asked. But, I've thought about this scenario, and I think I'd probably respond by asking, 'what do you think?' Family, friends and colleagues who know me, other than my wife (who monitors these purchases) and brother-in-law (who is a member on these boards), assume my watches are real, so they would never ask the question.
Based on the person's response to my question, I would try to assess whether they knew anything about watches and go from there. Ultimately, I think it comes down to whether the person believes that you could possibly afford to own a genuine watch, such as a Rolex. Most people around you will have no idea what a Panerai is or appreciate a "double red" sea dweller, let alone whether it's a rep. My wife thought my 217 was a "Panera" watch . . . you know, like the Panera Bread chain. Actually, for years, my wife refused to believe that my watches were reps (I've owned gens in the past and her watch is a genuine Rolex) - she thought I was telling her they were reps to keep her from complaining! Damned if you do, and damned if you don't!!