I've found that even if they say "oh yeah, I'd like to try that" or "that sounds fun" that the people bail when it's actually time to drop your drawers. Then you're left to swim naked alone :( Peace.
I think it depends on the person best way is to just use your judgement. I guess if you were swimming or something you could jokely say lets go skinny dipping, however it may not always work. If your friend was like oh my goss i thought of what we could do, just say go streaking. best thing to do is use your jugdement.