[stomachmonkey]

At it's simplest it's a file that is created the first time you run the trial which sets it's timestamp. All versions of the software will be set to look for that file and check the creation date against current date. If file not found then the software creates it. If found it does the math.

The file could be visible or invisible.

It most likely will get dumped in a directory that the OS depends on so it will not/can not change and will always be found.

Easiest way to find it is to do a search for all visible and invisible files with a mod/creation date the same as the install/first run date.

There are other ways to do it that are more difficult to get around.

You can always index the drive before an install then index after first run and compare the two.