Nope, no previews before the dld starts.
If you take the time to think about it, it's rather fairly simple and logical: you can't preview something you don't have and unless you start dlding the file, then you don't have it.
It's almost as if you asked: "Can I eat an apple without having one?"...
It's not impossible to do it, but it would need streaming and to do that in P2P would probably slow down everyone.