EPT
1) POSITION - Where is your opponent on the map? In scouting a 4-player map, you need to know where your opponent is. This is really the first and most obvious purpose of scouting.
2) GAS - Most important factor...generally tips you off to the build. If you do scout gas, how much has been mined? When did it go down? This can help you to understand if it was gas first and helps you count where the gas is going.
3) EXPANSION - Is there an expansion? This is INCREDIBLY important as an expansion means no early pressure, while no expansion means early pressure. In conjunction with info about gas, you can get a clear picture of what your opponent is planning to do.
4) BUILDING TIMING - It's important to know if your opponent went for a gas first or expand first or production facility first. Gas first always implies that some early pressure is coming, while an expansion first will indicate that no pressure is coming. It's also possible to scout something non-standard like a forge, 2rax, or 2gate. With the exception of FFE, these will all signal a rush.
5) MACRO MECHANICS - You SHOULD know where every protoss's chronoboost went. If you arrive early, you should be able to see how many chronos go down on probes, how many on the gateway or if he's saving it. This tips you off to 4-gates and let's you know if the protoss will be playing economically or aggressively. You SHOULD be keeping count of larva. If there are 3 larva sitting around while the spawning pool is being built, you have a good idea what's going to come out of those eggs. An orbital command should be morphing for terran at 15 or 16. A delay of that means there is a second rax.
6) FIRST UNITS - How many lings did the zerg player make? Did the protoss make a zealot or skip it and go straight for a stalker? Did protoss get a sentry second? Terran should always have an early marine followed by a second shortly after; any more than this should signal a 2rax.
7) WORKERS - This sounds ridiculous, and it's high-level stuff. But you really should be able to count the workers in your opponent's base. At a high level, this tips you off to a proxy barracks, how many chronos have been spent on probes, if a zerg is taking a hidden expo, etc.
If you can scout all this information in the early game, nothing should take you by surprise. If you're already taking in all this information, congratz! But if you're not, you can start at the top of the list and slowly work in more parts as your multi-tasking skills increase.
Again, this is to say how much information you SHOULD gather, assuming you're given free access to your opponent's base until the first units get out to chase away your scout. I understand that sometimes certain information, and perhaps ALL of this information is not important when doing certain builds. But it's important to know what you CAN scout if you did.
