Those who don’t know when to stop are already down to the addiction level, and gamblers that still have responsibility left in them can leave when they see the sign.
It's actually pretty simple to test if we truly know how to stop or not. Since we’re talking about bankroll management, and even discussing percentages, that bankroll should be decent enough to give us excitement without going overboard.
Now, the real test is knowing when to stop, especially when losing.
For someone to be called a responsible gambler, they should stop once the bankroll is wiped out, without the urge to refill or chase losses. Refilling after a wipeout is one of the clearest signs of addiction.