Marines can build an observatory that reveals alien locations within an area around it. Revealed aliens will appear as a red dot on your minimap. Observatories will reveal camouflaged aliens as well. I don't believe aliens have an equivalent.

Outside of observatories, you will have to rely on the line-of-sight of teammates and yourself. Both marines and aliens can see enemies visible to their allies, and also enemies that are attacking any of your structures. However, camouflaged aliens will not appear until they use an ability, and even the great Onos can camouflage, so be wary at all times!