Line of sight and elevated position

If in doubt, place the corresponding silhouettes and see if you can draw a line between the two.

 

You need to check the line of sight as well as you can.
For N3 rules, if the trooper on top of the building can see the trooper below it, the trooper below it will see the trooper on top by reciprocal LOF. Geometrically speaking you can't see prone units at a higher altitude than you without reciprocal LOF, though I think most people don't play it that strictly (I don't know of anyone who does).

http://wiki.infinitythegame.com/en/Line_of_Fire_(LoF)#Mutual_Awareness

 

This is one of the "trickiest" things of the ruleset of infinity. As others said, place the silhouettes and check, but remember you have to see at least a 3x3 milimeter square of the enemy silhouettes. I think this is why @Mahtamori commented about geometry, if the if the troop is "prone" on the roof and not in the exactly border of it, none which ist below it could see the 3x3mm square, maybe a 2x8 mm rectangle, but not the 3x3 required. As he said, often players "forget" about this restriction and do the shoot, or for simplicity, if your troup is prone and in the border you can see and can be seen, if not the model can't see or be seen (a lot of players who lived the N2 experience before N3 do in this way).

 

Yeah, it's because the base is 3mm exactly so if you're on an angle below it the floor that the elevated trooper is on will block LOF to a 3x3mm square. But a) no one really plays that was and b) mutual awareness usually makes this moot because it takes a special kind of rules lawyer to deploy a trooper on an elevated position with the rear towards the enemy and then argue that trigonometry prevents your opponent from seeing the trooper as long as that trooper doesn't declare a Move...

In either case, the 3x3mm requirement will probably(?) be gone in N4 along with the 1/3 requirement.

 

The 3x3mm requirement is in CodeOne, so it will also be in N4.

 

If we read C1 the thing with 1/3 it will be forgoted, but the 3x3 square still will remain, unfortunately, of course it will be an inprovement. If C1 translates exactly in covert terms into N4, then we will have covert when the enemy can`t see our silhouette completely :D (as it was in N2).

But if you are learning (or comming back) now N3, @Xagul82 don't pay too much atention on what we are talking right now. It will come in october :)

Edit... @ijw was faster than I.

 

Pretty much I normally see it played if the laser pointer can hit it it's fair game.