If it were real, you'd see the flash about a second or two before the crack. Fakers forget to take into account the difference between the speed of light and the speed of sound.
But that same difference in speeds makes actual video of the collapses a bit deceiving too. I used to ask myself why I don't hear any explosion at or before the onset of collapse of any of the buildings until one day I did a rudimentary experiment on a few videos.
I adjust the sound track for the approximate distance from the buildings that the videos were taken from, and low and behold, a totally different effect is achieved. I could hear a pre collapse explosion and suddenly it looks AND sounds like a demolition.
I think if someone with better info on where videos were taken from, and who had better equipment than I, did a similar experiment, it might be another major breakthrough in the case against the real perps of 9/11.