It's all about the gut feeling. If I feel a girl is acting, I do sometimes say so on the forum. And it doesn't help if the answers are like "you can't tell, people react differently". In that case, just take it as an opinion that it doesn't match my tastes. Doesn't make a difference for me. Some producers I am just not too fond of because the whole atmosphere doesn't feel quite right, everything seems a little artificial and the reactions often forced.
This is also part of the recent discussion about tickling clips being too "sexual". So far, my experience is, that the less "porn-like" the whole production is, the more focus is put on true ticklishness. And yes, there *are* really ticklish girls on clips. Situations where you just know: *That* can not have been fake. That's really the direction it works. Just compare the average clip by, for example, ZenTickling with the crap that TickleStaffer, the Realtickling follow-up, often (maybe not always) puts out. The first one just feels good, natural and most of the time the reactions look genuine. This difference *does* exist. Of course it is also about different tastes: Some people are more into the setup, they don't even care if the girl is acting. I don't get it. But hey, that's how many different fetishes are actually part of "tickling"...