You can pretty much tell when it's a fake.
Probably the beginning tickles, but after a few minutes, the 'tone' & intensity of the laughter changes.
I've noticed, especially the first few videos that were released back in the 1980s, that it was SO fake.
There are some women that are either great actresses or really ARE that ticklish.
Most of the Paradise Vision girls seem to be REALLY ticklish,
especially Jenny, Lily, Viviana, & Tracy, but to name just a few.
My only other observations are that they lose their ticklishness & that's when they 'cut' the action, then start again.
There is a video I have that Dorothy Laine is in. Most of the tickling seems kinda forced .. . BUT . . there were a few instances where she seemed to REALLY be ticklish.
I'll have to rummage through my vast collection of tapes & make an MPG of it to show you what I mean.