Play the opening of Vai's "For the Love of God", then just when they are getting into it, stop abruptly and comment about the guitar, "...the neck is nice...", etc.
You're the one spending your own money buying your ownn guitar, so you have every right to play whatever you wanna play, man. They laugh becos they are ignorant. There were times when I'm trying a guitar while another guy is trying out his at another corner, I just jam along to what he's playing all the while not knowing what song I was playing. If he's cool wif that, we'll just carry on, otherwise I just stop. That's another way to test guitar...heh heh heh...
Concern yourself with the quality of the guitar you're gonna buy, not what riff you gonna play, man. The people around won't pay you a cent for whatever you're playing anyway.