Do you feel like you have a good grasp of the game as long the opponent plays like you expect, but when they use an uncommon strategy you seem to fall short? If that's the case, you might very well need to practice scouting!
In the following article, I'll aim to answer the three following questions that I've heard people ask on several occasions:
Warcraft 3 is an incredible dynamic game with many nuances. This allows the players to chose from a wide variety of different strategies. Even though this is the case, there is a meta playstyle for each matchup. But what happens when your opponent deviates from the expected? Oftentimes an unexpected strategy can be devastating.
I recall losing to a Night Elf player who rushed Chimeras without building any other units besides wisps and his hero; Warden. It's arguably the most embarrassing game I've ever lost, but luckily those kind of games are the ones that you learn the most from. You see, my mistake in that game was that I let the Warden take up all my attention by harassing me, so I forgot to scout his base completely, and suddenly a fleet of Chimeras showed up at my doorstep causing me to forfeit in agony and shame.
The lesson I learned from that game is the importance of keeping track of your opponents every move so you (ideally) will never become the victim of any surprise attacks and unexpected strategies.
All the four races in Warcraft 3 have their own unique ways of scouting:
Undead's primary unit for scouting is the Skeleton Warriors produced by Rod of Necromancy. Rod of Necromancy is a relatively cheap item (150g for 4 charges) that you can buy immediately from the Tomb of Relics. The Skeletons can be used to keep track of the opponents whereabouts throughout the entire game. You'll also see Undead use Acolytes for scouting early on in the game. This is not because the Acolytes have special attributes for scouting, but rather just because you're able to scout with them before your hero pops out, which can oftentimes be very advantageous.
Furthermore, Undead has the arguably best tool for scouting in longer games; Shades. Shades can grant you an immense quantity of information about your opponent's whereabouts. A single shade following the opposing army can be game winning.
Night Elf utilizes their worker units, Wisps, for scouting. Wisps are incredibly good at scouting and there are several reasons as to why this is the case. First of they can work (produce lumber) even though they are nowhere near your base, which is a big positive and probably the most obvious reason as to why they are so effective at scouting. Wisps are also the cheapest worker unit (60g compared to the 75g of other worker units), which makes them more expendable, allowing you to produce more of them. Lastly there is Detonate.
Always having a Wisp (or several) near whenever a big fight breaks out can be huge. If you are fighting against Orc with Spirit Walkers you can drain their mana, so they won't be able to Disenchant your Druid of the Talon's Cyclone. If you are against Human with Water Elementals, you can hurt them severely, and if you are playing against someone who has found Scroll of the Beast or Book of the Dead, you have a great counter .
Humans have several means of scouting. In the early game, a single Footman will almost always be used to scout. Footmen are very expendable, and have the upside of being both faster and more durable than a Peasant. Footmen also have the advantage of being able to cause a bit of damage on its way by, for instance, harassing Wisps. Your early game creeping-speed will take close to no penalty as you'll be power creeping with Militia.
As an alternative to a Footman some people also use a Militia (the Peasant that finished the Altar). This allows for even faster scouting. Another way of scouting is to place Farms around the periphery of your base. This way you'll know when the opponent is closing in on your territory to either attack or harass you.
In the late game, you have several options: Scouting with invisible Footmen (with Sorceresses), using Arcane Tower's Reveal, Mortar Team's Flare or even the Mechanical Critters from the Arcane Vault. Reveal and Flare both have long cooldowns (180s and 120s respectively) and the Mechanical Critters are slow, so these are rarely used. Sorceresses' Invisibility together with a footman is often used, especially against Orc where you usually don't have access to a Work Shop.
I mentioned before that a Shade might be the best scouting tool in the entire game, but that might not be entirely true as a Shade is at least in heavy competition with Human's Flying Machines, which I haven't even mentioned yet. Flying Machines have little cost and low production time which together with their incredible speed (it's the fastest unit in the game) and long vision range makes for an insanely good scout.
Orc is a bit more unorthodox as far as scouting goes. In the early game, they have no normal units that are good at scouting. Grunts are too expensive to use solely to scout and Peons gain no additional benefits when scouting like a Wisp does. Instead Orcs use their heroes. Far Seer can use his wolves to scout in addition with Far Sight (though that spell is pretty much never picked). If you've opted for Blademaster, you'll often spend a lot of time on top of your opponent's army trying to creep-jack and harassing them with vast power that Orcs have in the early stages of the game.
When entering the late game, Orcs can also make use of Witch Doctor's Sentry Wards, which in large enough quantities can light up the entire map as showcased in this game by Grubby:
This is definitely the most asked question among new players, I encounter, with the topic of scouting. Unfortunately, it's also a very hard question to answer as you have to keep your eyes out for a lot of things.
In the early game, you should keep an eye out for the enemy's
In the late game scouting mainly revolves around scouting
As mentioned, there's a ton more things that could be important to notice than what I'm able to cover in this article, but here's some of the specific things you could encounter and should be looking for when playing against the different races:
Versus Night Elf