The question is how do you define the trigger(s) for applying the runaway stabiliser trim drill? STS, MCAS and the AP will (and do) adjust the trim without pilot input, so what exactly are you looking for? Also, this is not on a low workload flight deck level at 20,000’, this is shortly after takeoff in a critical flight phase where all margins are much smaller and spare cognitive bandwidth is much reduced. It needs to be simple logic, not a large branching flowchart.
I guess what I’m trying to say is that suppose the trim starts moving after takeoff: is that a runaway or is it normal operation? I can’t see a quick and easy way to figure it out ...
Very easy to figure it out. This happens by itself when it dawns on you that the one thing you are fighting with the aircraft over is the pitch trim. There may be a few annunciations here and there that you haven't seen before (and a stickshaker that started whilst still on the runway), but everything else about the aircraft seems to be working normally.
Again:
The nose attitude keeps wanting to go low. You keep applying trim to make it go up. You make a trim up input, but the aircraft then trims nose down. You are spending a lot of time and effort on trying to trim for nose-up, but something keeps trimming nose-down. Pretty easy to figure out.