You could either make a video of yourself to show them, or go and play a few for free and then use them for references.
I wouldn't do more than 30 minutes free though.
That's so when you use them for a reference they won't say, "oh he did an entire show for free."
Years ago, I used to tell them my price and then say if you don't like it, you don't have to pay me. I never got stiffed.
DonM