Frustrated male tickler, a redundant term
I agree with "Grippedchimp" - if you know someone's gonna tickle you, it is easier to resist than the unexpected tickle. So if she's lying just to challenge you, try a sneak attack.
Does she know you're a tickler - and is that why she's goading you? Or have you simply tried to tickle her, only to get no reaction and hear her say those frustrating words, "I'm not ticklish."
Most times girls told me that, they were telling the truth. But I can think of at least one who lied. I brought up the subject of tickling with Patty and she said she was not ticklish - but later I reallized she only said that so I would not tickle her. So give it the old college try with your female friend!
Then there are the ones who are ticklish but good at resisting it. Once again, you just have to keep trying. The one girlfriend who turned the tickling tables on me was an example of this. Georgette always tickled me but my attempts at retaliating failed. But one time as we wrestle-tickled on my sofa, I finally got her - I stood, held her legs up (lifting behind her knees) and tickled her on the bottom thighs/rear end. Panic covered her face and she begged me to quit it. "I'm not used to that," she said when I stopped tickling her.
So maybe this female's telling the truth. It sounds like she enjoys taunting a tickler; I've known a few non-ticklish folks who loved to tickle because you could not get them back. But if she's lying, you may need to either 1)surprise her, 2)find the right tickle technique (soft tickles, hard tickles, whatever) or 3)find her elusive ticklish spot - supposedly the underarm area is where more people are most sensitive, although with that tease Georgette, it was her bottom thighs and rear end. Good luck and let us know how you do!