Yea, when I first got out of the Army (slimmer w/ high&tights) I would always get the "you a cop?" question when I went to parties. The fact that I always have a serious demeanor, don't drink, don't smoke, don't do ????, just made it worse.
I almost got into a fight one time with a drunk guy who was also smoking pot in a backyard BBQ. He offered me a toke and I said, "nah, I'm cool man" then he looks at me funny and says, "you a cop?" Despite my numerous denials the guy just would not leave me alone, badgering me all night about showing my badge. I was at the point where I was ready to leave when the dude faceplants into the grass and falls asleep.
The funny part is that I really look more like a thug than a cop. If you ever watch The Shield and see the extras they use for gangbangers, well I look like one of those guys sans tats. I'm so mean-looking that my boss asked me to escort one of our female workers to her car every night after work for a week. Another co-worker was harrassing her and my boss thought the guy wouldn't try anything with me around.