Just be yourself there and see if you like the person... on a double date it will probably not go any further than talking since there is no privacy. Perhaps it's better to see it as an opportunity to learn to know someone than a real date? A face-to-face date would be the next step I guess :D
My boyfriend go out on double dates with a friend of mine and her boyfriend occasionally. I find them fun! We go, eat, my friend and I will gossip most of the time while the guys talk about guy stuff, etc.