Problem with pushing the direction button on the remote is that it's a function built into the electronics, so they will behave as they are programmed to do, which on MTH PS-3 running in DCC is to slow down and then speed up in the other direction. Legacy will almost come to a complete stop with very minimal coasting. So to run your test you truly need to cut track power.
Yep, the only way to verify is to cut track power.
Unfortunately, we are at the mercy of the control electronics in terms of the resulting behavior. As I mentioned above, when I cut power while running on analog AC, the PS3 board stopped the motor immediately. When running on DCC, it ran for almost 10 seconds.
On the ESU Loksound decoders, for example, this behavior is configurable up to 4 seconds, but with the stock PS3 (or other!) electronics, we have no such option.