Another way I used to do it..
I mentioned how before I joined the forum, I had a close friend/pen pal who was into tickling, who passed away. He and I used to ask all of our female pen pals, what we used to call "Our Favorite Question". Many times, we got answers. In my case, from dozens of girls, in snail mail letters, before my friend died, and I joined the forum.
Anyhow, to how I used to do it. I would, after a letter or two of normal conversation, discuss how I had been tickled in the past, and then ask the girl if tickling had ever happened to her, and if she was ticklish. MOST times, but not all, they answered, because, I posed the question in a seemingly innocent way, and not all.. ""I want to find out if you are ticklish, to get my jollies".
I don't know if this would count as "discreet", but this is how I used to go about it, and it worked.