You build ships, fuel them up and send them into deep space with their sensors set to long range. If you have a small cheap ship that can hyper jump have them jump as far a possible scanning for the enemy. Scattering out a few ships with long range scanners in deep space would also be a great help in locating any would be trouble makers entering your area of space.
It is a good idea to build bases on planets between your homeworld and where the enemy might be. Build scanners on the bases to watch for enemy ships flying by.
Jan 28th, 2004