It’s impossible to tell if you are going to actually kill a dino because when you hold your attack, the opposing dino’s HP bar only drops by your attack power. It doesn’t take into account shield, armor, damage reduction, damage increase, etc. We can’t really be expected to calculate that on our own. Especially since we don’t know the order of which all of these are applied.
When you hold your attack and the game says the other dino is going to die, it should die.