In the classic version of War2, you could find an invisible unit by clicking on the map where that unit was. You can use a huge mouse drag to see if an invisible unit is on the screen. If one is there, you can narrow it down by dragging over smaller areas until you have it isolated. Beware, dragging over a unit or building of yours will take priority over the invisible unit, and you will only see your unit/building highlighted on the left side of the screen. In this picture, I have my mouse right on top of where this invisible mage is. I brought the mages icon that appeared on the left side of the screen after clicking on it, to here, to show you his current health. That poor invisible mage thinks that he is safe.
Wrong! Because I know the area he is in, I can still find him with all sorts of spells, or attack ground commands. This time I used coil on him, as you can tell by the mages new current health level. You can find ANY invisible units this way in Classic War2, as well as Submarines, and Giant Turtles. I find it very interesting that Blizzard actually fixed this in War2 BNE. This didn't seem like something that needed changing in the new version, compared to other bugs the game has. I guess it was just easy to fix for the coders. Although you can't find the enemy with this trick in War2 BNE, you can still hit invisible units with spells. If you ever want to guess, coil is a good spell to use. If you hit something, you will see a coil shoot off in that direction. Coil also is a little forgiving and will hit a target even if you click up to 3 squares off of the enemy. If you hit nothing with your cast, you will lose 100 mana for nothing.