Bring the subject into conversation

Bringing it innocently into conversation with someone and letting conversation flow from there... How do you best go about?!

when you talk give them a little tickle ... see their reaction... say oh your ticklish or aren't you ticklish ? no matter what they say you can say Is that fun ? or do you hate it ?

Well, what I like to do when I am having a fun conversation with a friend and the opportunity presents itself is ask a "situational" question. For example:

Her: I was so tired. I slept really good last night. I was knocked out.

Me: Oh yeah? You were knocked out? Like out cold, deep sleep, where you drool all over the place type sleep? haha

Her: *smiles* yeah.

Me: Are you so knocked out, a bomb can go off and you are still sleep?

Her: Oh yeah. haha

Me: What about if I were to lightly tickle the bottoms of your feet while you were sleep. Would you wake up?

Whatever her reaction is to that, you go from there. When I have asked that question, I get good responses. Depending on their reaction, I either let the conversation continue a little more, or I kill it after that and talk about other stuff. Do not go overboard with it though.

When getting to know a prospective romantic partner I might ask them if they have any turn-ons or kinks or hard-limits & use this as a lead-in to discuss mine.

Flirt with her. Make her laugh. When she leans sideways to bump you with her shoulder, tweak her side; then, when she laughs, say, "Ticklish, are you? Well, now, you're in for it!"

Here's a variation:
On dating apps I've sometimes said, "Want to play 5 questions?" I tend to make my question #3 pretty substantive, like "What's something you've done that you regret?" Whatever they say I'll respond with something thoughtful and accepting, with some self-awareness thrown in. That perfectly tees up my question #4, "How ticklish are you on a scale of 0-10?"

It's works first because it's in a dating context, but also because it's placed in a list of questions, and I usually earn what they consider the silliness or intimacy aspect after having just discussed something deeper. If I just asked it as a non sequitur, I think most of them would find it strange. Also, when they give a number for a response it fosters better additional back-and-forth than a "yes" or "no" would have.

Honestly, lately, the past few times I brought it up with coworkers and friends, I usually use my fiancee as kind of a scapegoat. I basically find a way to bring getting your nails done and pedicures into the conversation.

Sometimes I'll compliment a girls nails and then let it drop that my fiancee "forces me to go with her to get my nails done". Then I'll say something about how she's extremely ticklish and it's really funny to watch her kick the pedicurist. 9 times out of 10 for me the girl either laughs or agrees with me, and sometimes even admits she has the same problem.

If she doesn't admit it, I'll attempt to bait her a little bit by saying something like "can you relate to that?" Or "I bet you don't have that problem, huh?"

Lately I've been unlucky though. The past couple girls I asked both denied having ticklish feet. The one girl admitted she isn't ticklish at all and her boyfriend tickles her all the time, and the other one said she isn't ticklish, at least not on her feet.

But I'm careful not to push the issue any farther. There's only so much you can do before the conversation crosses into weird and creepy.