One thing I've always struggled with in RTS games is scouting. What are some effective methods of scouting what your opponent is up to in Starcraft 2, especially in the early game? How do you accomplish it without losing focus on what you are doing back at your base?
Certain units are better at scouting than others, but you should always scout constantly. There are a couple units which work well for scouting:
- Early worker -> a definite must, usually at ~10 supply, but if you think you can do it around 12-13, even better
- Tier 1 units (marine, zergling, Zealot) -> they're cheap so suicide them into your opponents army to see what he has been building
- Air scouts (Overlord, Observer) -> you may have to sacrifice these for intel, so I suggest you place one at the top of the opponents main, and one at the bottom. This way you can move in and quickly see his entire base
- Terran specific (Orbital Command and Flying buildings) -> I actually recommend building an extra barracks and flying that in to scout. It has a lot of HP and should see plenty. Also, every MULE is worth 50 energy and gets you 250 minerals. Usually a better use of 50 energy than a scan (though not if you need to see invisible).
- Protoss specific: In the expansion Heart of the Swarm hallucinate no longer needs to be researched and is a fantastic way to scout. Hallucinate a phoenix and fly it where you want to scout.
Goals of Scouting:
- Scout a build order (opponents starting production) -> When did he build a spawning pool? Did he go 2 Gate or Cybernetics core? Techlab on his Barracks or Factory? These will help you deal with early agression
- Scout attack forces -> Knowing when an enemy is coming is vital. The more time you have to prepare the better. Place a unit near his ramp and control those watch towers if you can!
- Scout composition -> Knowing mid/late game what your enemy forces look like, will allow you to better adapt. Lots of Roaches? Build some immortals. Tons of Marines? Morph some banelings. Colossus? Vikings!
- Scout expansions -> leave some Tier 1 or spare units at various expansions on the map so he doesn't sneak in a free base.
When to Scout:
- Every chance you get
There is never a bad time to scout.
Note: I would caution against assimilator trick. While it is slightly better as protoss than other races, you are essentially wasting resources and forcing him to play a more Tier 1 focused and aggressive strategy. If I only have minerals I'm going to build a lot of Gateway/Barracks/Hatcheries and push out cheap units instead of teching. Not a good position for my opponent who had to sacrifice 150 minerals.
In addition to other great answers, a Protoss-specific scouting technique is to research hallucinations, then create hallucinated phoenixes and send them over the enemy's base.
They might be fragile (hallucinations take x2 damage) but they're as fast as any other phoenix and they only cost energy, not resources. You can get them decently fast, faster than the earliest possible observer, and this method can remain effective later on.
I know some players prefer to hallucinate probes when looking for enemy expansions. Inferior to the phoenix, perhaps, but as you can create 4 from one cast you can send each to a different area to check it out.
Finally, while using hallucinations solely for scouting may seem like a waste of the research costs, hallucination is actually an underestimated but useful ability. Besides scouting, good uses of hallucinations include:
- Bolstering your army with false units to scare the enemy away or at the very least act as meat shield, causing the enemy units to waste attacks and abilities on these false units.
- Tricking the enemy into thinking you're going a different tech route than you actually are (especially useful with colossus, void rays or high templar).
- Trick the enemy into thinking you're executing some specific tactic such as a rush (hallucinate lots of zealots) or a drop (hallucinate warp prisms).
For really early scouting, I hotkey the worker to 1 (select it then ctrl + 1). I take advantage of the fact that you can hold down on shift to queue up commands and usually just hold down on shift and right click a couple laps around their base and then right click back on mine. Not the most pro way of doing things, but it keeps me from having to worry about micromanaging my scout while keeping my build order on track.
For terran/protoss, I usually send the worker that finishes the first supply depot/pylon. For Zerg, I usually just send my 10th worker if it's a large map and my overlord can't find them quickly.
Scouting later than that is a much more involved question and largely depends on the circumstances, race matchup, map, etc.
Early game, all you need to do is send a worker out. If you're early enough, you can get full view of their base before they have any combat units out, forcing them to either take a worker off their mineral lines to chase down your worker, or give you unmitigated access to their base.
Don't worry about micromanaging the scouting worker -- send it out, and then ignore it until you see on your minimap that it's near your opponent's base. Rally various locations within your opponents base so that your worker-scout is continuously moving and can't be chased down by another worker, return to your economy, "peeking" at your opponent's base as needed until they get a combat unit out and destroy it.
If you're playing as protoss, you may even be so bold as to put down an assimilator on your opponent's gas. It's only 75 minerals, (so only 25 more than if you left your worker to be destroyed) and is twice as hard to destroy compared to refineries / extractors (due to it's shields)