Instructions
Familiarize yourself with each race's detector units, their cost and building requirements. Each race has one stationary and one mobile detector unit.
Build a defensive perimeter of stationary detector units around your entire base. Ensure this perimeter has no gaps in it, with the line of sight of each detector overlapping slightly that of the nearby ones; pay particular attention to the areas from which you're most likely to be attacked.
Build several mobile detector units and keep at least one with each group of units you send out. Doing this ensures you always know exactly what you're up against before you engage your enemy.
Replace both mobile and stationary detector units if your enemy destroys them to ensure you are never left unable to identify cloaked units.