You can't. It just happens to be a question that is weird and random in most cases.
However, you can have some success if you just ask "Are you ticklish?" and maybe preface it with "Silly question".
When you start getting specific and in depth with it is when things start to become creepy. Ex: "On a scale of 1-10, how ticklish are your feet?" "What are your most ticklish body parts and rank them on a scale of 1-10" "Are your feet ticklish?" etc. That's what you see a lot of people do on Twitter, and they come off as creepers.