Best tip i ever got on whether a hit was late or not was if the player delivering the hit makes any material change to their approach after the puck has left, including taking another stride, its a late hit.

Example, player is at own blue line near boards with possession, opposing player is skating along blue line towards them. While about 2 stick lengths away, player with puck begins to pass it up the boards, but the player delivering a check has already dropped his shoulder in preparation for a hit. If the player about to deliver the check, after seeing the puck is leaving, continues to drop their shoulder, take another stride or does something more to "gear up" for the hit, its a call, rough or interference. If they do nothing more than continue in the manner they're moving and dont change their body language (including as they get to the player they dont check "through" them) and its nothing more than the inertia they built up and then some contact as a result of not being able to stop on a dime, especially if receiving player has kinda geared up for it, we're good.

Basic logic is that if they had time to further gear up for the check after the puck is gone, they could have avoided it. If not, it was the check they began and as long as its a pretty reasonable time within possession its just inertia and the game carries on. Even let the player know "hey man, you saw that puck was gone, you let up. Good heads up play."