It's almost impossible to prove a 'negative'. Although in our legal system, a defendant is presumed innocent until proven guilty. Therefore, it is incumbent on the accuser to prove the defendent had 'access' to the material. Therein lies the argument.

In terms of 'script theft' if I can prove that the company had a copy of my script,(by virtue of my retern receipt for delivery) Or PROVE tha I gave a copy to John, and John gave it to Marsha, and Marsha works for that company... than it will be hard for them to argue they didn't have access.

If the book the script is based on is widely distributed... (A bestseller) it will likewise be hard to prove that the defendent DIDN'T have access.

It's one of those things thats extremely difficult to prove (and ends up being central to the question of most of these type of lawsuits) and depends entirely on the attornies and jury involved (or judge, if no jury is invovled).