This is my how to on build orders. If you find this useful please feel free to bump it with a post or thumbs me up.
Alright, so today I'm going to talk about scouting.
First, lets take an overview of units that terran scout with. SCV, reaper, marine, hellions and OC. Additionally you can scout with a barracks/factory and air units later in the game.
When to scout: I usually like to send the SCV that built my supply depot off to scout. while he is building you can rally point to different areas by holding shift and then clicking so you can set up where you want him to scout. If you're planning to cheese send your first SCV that you create, if you're really weary of cheese send the SCV right after the one that starts building the supply depot. So really you either scout on your 7th supply, your 10th supply or your 12th supply.
Now to break scouting down in the game.
Early: Scout your opponent with an SCV, then either build a reaper, build a barracks near his base to float into his base, or comm sat him to see what he is going. The cheapest is the reaper, the most cost effective is the barracks, and the fastest is the comm sat.
Mid Game: Air units or an scv/marine/hellion need to be scouting expansions. You don't want him hiding an expansion. Comm sat and floating builds are the only way you're going to get to look at his base at this point, if you choose the building you will lose it however. Still cheaper then the comm sat. Use an SCV or a marine to scout his army, you can see his composition, placement, etc... and although you'll lose 50 minerals it's no biggie.
Late Game: Comm sat is still your best scout. SCOUT EXPANSIONS, do not let him get a hidden one. If he gets an expansion you need to know about it. Also scouting his army and it's positioning is very critical at this stage.
Now, why is it important to scout? Well, for two reasons. You want to know what your opponent is doing. Is he building hellions to harass you? Does he have a phase prisim to warp in units? What about surprise DT's? If you know what he is doing you'll be able to effectively combat him. Second, you want to know when and where he is expanding. You need to know this because you can either choose to expand or choose to punish him for expanding. One way or another he is spending 400 minerals on that expansion so you will technically be "up" on that expansion, not to mention the cost of workers to saturate it.
Finally a list and overview of units the other two races have to scout with and how to stop or deny scouting.
Probe: Just like the SCV roll for us the probe has the same roll. Without compromising your build they will nearly always scout you early, that is ok though kill the probe before revealing your hand. Later map control plays a key roll in denying scouting for expansions. Also not keeping your whole army at the front will deny scouting for unit composition.
Observer: Ravens or missle turrets and a viking or a few marines handle this easily, and since observers are a hefty 100 gas it's well worth it. I highly suggest incorporating a raven into a build vs. toss in order to eliminate their map hacks observers.
Phoenix: This one is hard to stop, it's very fast. The only real thing you can do is hope it flys over your army and have enough marines to 1 shot it. Really just an undeniable scout.
Drone: (Read probe)
Zergling: (Read probe) These guys will often hold the watch towers too. Before you move out send a marine or two to clear the watch towers so as to not tip your hand.
Overseer: When you hear sac an overseer it's basically the same as what we do with our floating buildings. Keeping a few marines at the edges of your base will help dial this down but it's really just impractical to stop this all together. The best way? Vikings. They'll either force him to recall his overlords or kill it before it sees anything.
Overseer: Just like an overlord but faster and healthier, really it's unlikely you'll be able to stop him from scouting. Just make sure you kill the changeling when he drops it.
Liked this thread? Find it useful? Please give me a thumbs up and post on it so that others will see it and it gets bumped. If you have any questions feel free to ask and I will do my best to answer.
This is Beammeup signing out.