Outposts are marked on your map with a red flag these are small compounds with a group of guards some on the ground some in elevated positions with sniper rifles each compound will vary. To clear these quick a good strategy is to press up on the d pad as you approach zoom in and out with the triggers and when you see and enemy tag them by pushing rb than you will see their movments on your radar and you can take them out with stealth. Another good tip is to find the alarms switch in the compound and sneak up to it and disable it by holding x, this will avoid reinforcements being called one the bullets start flying