If in a relationship, just ask. If she has a hint of maturity, there will be no fuss, u wont be branded a freak, and will just get a "yes" or "no". It might need a little integration to be fully comfortable with the idea, but it just takes time.
I thought it was kinda wierd to like being tickled, and to an extent I still do, I'm not fully comfortable with the idea. Yet I get tickled on an almost daily basis and some heavy bondage tickling every once in a while.
What I did, was when we first were seeing each other I got her to do a back massage, which always involves soft and heavy touches. I responded to the light touches more than the heavy, so she began to do that more. That way, your partner sees you enjoy the lighter, and massages tend to move to new areas of the body.
I mentioned to her that I quite liked it. However, it took almost 9 months to make it quite clear just how much I liked it. She knew I liked it and did it quite regularly, but didn't understand just how much it turns me on, and how much fun I found it. I thought the positive responses and the request for bondage made that clear, but sometimes you just gotta put it plainly.
She loves tickling me because I enjoy it, the same with anything else we do. mentioning it will give you something else to do. She doesn't enjoy being tickled much (well, depends, slow tickling is nice when she is in the mood or relaxed), but none of the fast stuff. Shame, good job im primarily a lee then 🙂
Remember; your partner doesn't have to be into it to do it, its not a particularly extreme fetish (no offense to anyone, but I consider golden showers somewhat extreme, at least far more than tickling). Tickling is far easier to enjoy for most people, although perhaps not if you are a ler. I think iluv2tkl has something about integrating tickling if you are a ler in his big post, i cant remember (i read it somewhere), about assossiating tickling with positive feelings. Go and read it 🙂