The playful wind up/retaliation thing is for sure an option.
If you play games, try Twister (tickling is a playful way of cheating), or computer games (let them beat you and then use tickling as a way of putting them off, a la Distraction).
If you want to avoid actually trying to tickle them, get a conversation going about reflexology/pedicures/fish spas/massages, Mr Tickle (from the Mr Men books), medieval torture methods etc - anything where tickling/being ticklish can be a relevant part of the conversation, so there is context for you suddenly asking "are you ticklish?", or simply admitting that you're ticklish yourself and seeing what response you get.
Or talk about injuries, and invent a story about a time you got/gave a black eye/bruise during a ticklefight, and again see what response you get.