How do you spot enemies in Gears of War 3?

While looking at an enemy, hold down the left trigger button (as if to aim your weapon) and click down the left stick. Your character should then say something along the lines of "enemy spotted" and a symbol will appear over the enemy's head.

Additional information:

• Spotting an enemy in Horde mode will earn you a little cash if that enemy is subsequently killed.
• If you are killed in Horde mode, you can still spot enemies for your team mates by entering ghost mode by pressing X (which enables you to float around the map) and then clicking the left stick to spot enemies. This enables you to earn cash to use in the next round, or to buy yourself back into the round if you earn enough.
• Weapons and ammo can also be spotted using the same technique; if you spot 3 weapons that are subsequently picked up by team mates, you will earn the Reconnaissance ribbon.
• You can earn the Military Intelligence ribbon by spotting 5 enemies that are subsequently killed.
• Spotting enough enemies will earn you the Spotter medal.