Usually, audio or video recordings, or documents (emails, tweets, paper letters) in which the suspect describes his mindset and motive.
For example, if there was a recording of Trump stating, "Well, we want to quiet these women down before the election -- this stuff might impact my chances..." that would be pretty damning material.
That sound familiar... doesn't it seem like we have a lot of those regarding the nature of the witch hunt?
I am reading That there are claims that the prosecutors now actually have this corroborating material that describes just such a purpose for making these payments that is to say, influencing the election. This is from articles that quote the Southern District of New York prosecutors.