How did he know? he was lunging forward and probably had a feeling but he could have taken the chance of staying on and be adjudged not out.
If you play enough, then you can just 'feel' that crease line, and hence it's often quite clear that you didn't get back in time before the stumping.
On topic, if it's pretty obvious (a nick, run out, stumping), then there's no real reason to stand around. Even if the umpires miss the edge, the honorable thing to do is to walk off. Don't think Maqsood even walked off the field entirely, I believe he was just waiting close to the boundary lines (from what I could tell). If it turned out that he wasn't out of his crease at the time of stumping, then he'd be simply recalled.
If it's a dodgy catch, or an LBW, then it would make sense to stand your ground since you're genuinely not sure. Haris should have stood his ground and asked the Umpire about the bowling mark indicator that came off Saqib's run up, as that clearly distracted him and could potentially nullify the ball as a dead ball. Doesn't matter if it doesn't affect the end result, it's about principle.
But I'd say that's pretty irrelevant either way, since the informal protocol nowadays is that the umpires verify the dismissal regardless of what the batsman under consideration perceives it to be. And since the rule of 'not reversing the call or not recalling the batsman once they cross the boundary lines' has been redacted, you can always be recalled even if you're sitting in the dressing room, and the next ball hasn't been bowled (iirc).
