I do not think Crooks knew no one was posted on the roof. He showed up hoping it was clear. It was and the rest is history.
I would strongly encourage you to watch this fifty minute video by some excellent pros on this subject.
https://www.youtube.com/watch?v=CUkgNChQ140
We now know a lot more than we did even a few days ago—from multiple sources—one cell phone video proves Crooks could not have fired the shot that almost killed President Trump.
That particular piece of data is about three quarters through their analysis.