First would be common sense. Teams always looking for edge. Teams filming for decades. Teams actively stealing signs forever.
Second would be those in the game saying so... like the quote above from The Athletic. Or Tim Flannery (former Giants coach) saying how they had a camera trained on opposing 3rd base coaches every game. Yes, this is 'more' than what Flannery was doing, but it's just another step.
I think this is several steps beyond anything you're describing. Murder is several steps past having a temper tantrum at someone too. Those steps matter.