I hope this helps, ticklejen.
Before I say anything else, I have to admit (as I did in another thread) that I have never tickled anyone for more than a brief period, and that I had never met anyone as into tickling as I was before joining this site a little more than a month ago.
Still, my life has not been barren of tickles.
One of the ways I test the waters and get in a few pokes is when a guy I am seeing and I are teasing each other or joking behind closed doors. (I am talking about M/M, but it may work with F/M as well.)
I think I have done this four or five times over the years (mostly when I was younger):
If he is teasing me verbally, I often say something (very playfully!) like "Watch it, buster, I can make you suffer" and then do something like poke his nose (which, of course, does not tickle at all but does add to the general silliness of the mood). I might also give him a very, very light play slap with the back of my hand to his forehead, brushing up the front of his hair in the process. All this is calculated, of course, to get him smiling and perhaps even laughing (pre-tickle). I never know how ticklish a guy is or isn't, but I do know it works better when he is in a good mood.
My "watch it" remark is often met by a "Yeah, right," in which case I may say something like "Don't be too sure" and continue the playful banter. A few pokes in the ribs will certainly follow--especially if (ideally) he is grinning and saying that I cannot make him suffer.
Some guys are not into tickling at all, but I have never had anyone get mad at me since the situation in which I test the water is already goofy and playful. In addition, I am talking about two or three pokes, not full tickling.
Now I am thinking of H and M, two men from my past who were deliciously ticklish, something I found out in this very scenario...