I'm all about consent as well, and that is a tricky subject. It's weird because my ex and I used to tickle ALL THE TIME but when we first started dating and I tickled her for the first time, I was in the same situation you're describing. I ended up just tickling over her shirt because I didn't want to cross the line of any boundaries. She actually wound up pulling her shirt up. It turned out she wanted it.
If the situation didn't go like that though, I'd say maybe if they're really ticklish just tickle them big time over the shirt in spots that really make them jump and squirm, and as they're doing that, and it's a real possibility their shirt could ride up on its own as they're flailing around. If they don't pull it back down, I'd say take a shot at the skin that becomes exposed and then play the situation as it goes.
