ORIGINAL: Steve McClaire
The AI units spot player units exactly the same way as the player's units spot AI units. The AI can spot muzzle flashes from firing units and figure out where they are -- this is not guaranteed and is not substantially better than an experienced player in my experience. One advantage the AI has is reaction time. It notices instantly if a new enemy unit has been spotted, whereas a human player may need a moment to notice the new blip on the mini-map and move the view to see what it is.
Steve
I don't agree Steve. Then how can the AI instantly spot a unit when you order a "fire" order and EXACTLY the same time, Before any muzzle flash, target the unit?