I never *just* ask. I want to develop a friendly relationship with my lee, not just any random stranger. So without at least some semblance of social chat and get-to-know-you's, I will not just flat out ask.
Tommy from Tickle Abuse once stated here that he does not mind other people's perception of him as a tickler; but he cares a lot about where people first hear tickling from. I totally agree with him. How someone first hears about tickling in a fetish context, and whether the message was delivered in a non-threatening, comfortable, friendly fashion, goes a long way in helping them determine whether tickling is something fun, or its a creepy fetish.