I'm never quite sure of my level of attractiveness so I've always wanted to go up to a random good-looking fellow and ask him.
Most Helpful Guy
You could do that, but the likelyhood that you would get a straight answer wouldn't be very good. He'd probably tell you he thought you were attractive no matter what, as unless he was trying to be intentionally rude, he'd probably not tell you that he didn't think you were good looking (if that's what he really thought).
If you came up to me and started hitting on me and asked such a question, if I thought you were attractive, I'd tell you. If you were just asking matter-of-factly "hey, do you think I'm attractive, I just need to know and I'll be on my way, I'd just be honest if I thought you were average or beautiful, one way or another.
Going up to some random guy in person isn't going to merit you very good results for accuracy, as it is only one guy. Also, if he thinks you're flirting with him, he might not tell you he thinks you are only average if he wants a chance with you. What girl doesn't want to hear she's the most beautiful thing ever? The guy might lie out of self-interest.
If you really want an honest opinion, you should upload a picture here on GAG and turn this question off fro anonymous and post it with your actual image or a link to a image upload of yourself. You'll get far more guys to answer and a better and broader idea of what guys think of your looks.