Difficulty with Inverted Conditions

To keep it short, I've been messing around with the free version, making extremely simple games just to see how I like the program. In my current game I'm working on, I'm trying to make it so that when all entities of a specific sprite are gone, the player get's a game over popup. I've noticed there is a 'see if on screen' condition, so I tried to use that for the sprite that the player goes around and destroys. I inverted the 'see if on screen' but it seems to do nothing different, and I can't seem to find a way to make this work. So I suppose my question would be, how can I make it so once all entities of a certain sprite are destroyed, a condition is fulfilled making another action occur? Also, what exactly does the 'invert condition' do? I was under the assumption it would be the same as a 'not statement', but that doesn't seem to be the case.

You could check the sprite.count to see how many are left.

Under what condition and action would that expression go into though? I'm trying to get a better understanding of the event system, but I'm not sure where specific expressions would go, and how I would run a 'check' of something, or an amount.

System -> compare two values

Ah. Well thank you both very much, I appreciate the assistance.