I don't believe you need to make your tickling inquiries discreet. In most chats I've seen, the one who asked the question about whether the woman is ticklish just came right out and asked, "Are you ticklish, _______?" or some other variation. I guess you could just ask something like, "If you were tickled..." if you don't want to be too direct. Just my 2 cents, or whatever the rate for advice is nowadays! 🙂
I usually acknowledge that the question is a silly one to begin with, and just for fun. Polite, too. Not creepy. Definately not creepy. Just don't be creepy and all will go well.