This is the first entry in a small series of posts I plan on writing which involve me articulating the knowledge I've acquired on my journey from bronze to platinum. I doubt high level players will find anything of use here but I hope other lower level players will find some of the information useful.
Anyways here is the first post and it describes my scouting practices, what I look for, and how I respond to what I see. The scouting is very basic, mostly just keeping track of expos and recognizing when anything non standard is occurring.
Scouting Protoss
Overview
Protoss has a large variety of builds that allow for early aggression. Most aggression can be defended by adding a bunker or two and quickly repairing; however, some of the tech heavy all ins are nearly impossible to hold without proper preparation.
When your original SCV scouts a Forge Fast Expand or 1 Gateway Expand
If the Protoss forge fast expands or 1 gate way expands most likely your original bunker will suffice. However, make sure you scan around 8 minute mark to see if you opponent is teching up to an AOE tech (look for a robotics bay or templar archives). If they are not and have a shit ton of warp gates expect a 2 base all in and add another 2 or 3 bunkers.
If there is a Twilight Council but no Templar Archive its probably blink stalkers. I've died to all such all ins where I didn't retreat back into my main. You need to minimize the attack area and use the SCVs from the natural for tanking or you will get picked apart. Be sure to keep pumping out units and try and snipe the observer if possible. If you hit the critical mass where you can engage his army directly despite the blink advantage you win.
This is basically the 4 gate but powered by a strong economy, resulting in a very strong attack of gateway units and the economy to support multiple warp in cycles. If you keep on top of you macro and make sure to get the repair on your bunkers you should be just fine. Also if you survive its a guaranteed win since MMM vs gateway tech isn't much of a competition. March over to his base and roll his gateway army with your bio ball.
When your original SCV scouts no expansion.
If you scout no expansion hide your scv so you can poke back in later and see what he's up to and go ahead and add a second bunker. As a general rule about every 2 minuets a Protoss stays on 1 base I add another bunker. So if the attack hits at 5 mins I have 2 bunkers, if it hits at 7 mins I have 3 bunkers.
I try to sneak my scv back in after I finish my production buildings. If I see 4 gates I stay on 2 bunkers but preemptively pull a couple of scv, if I see 3 gate robo but no immortals I pull a couple of marines to the high ground to try and snipe any warp prism play, if I see immortals and sentries I add a 3rd bunker
If the Protoss has not expanded by 8 minuets and not attacked yet scan his base. Mostly likely there is a colossus involved. If you see a robotics bay or a colossus skip your first 2 medivacs and go strait for vikings and add multiple bunkers.
Basically when I play against Protoss I am trying to recognize if the Protoss is deviating from standard macro play and if he is I scan to see when it will hit and what the composition of the attack will be. Build an adequate number of bunkers and get good at scv micro (attack to tank then hold position to block) and you can hold most all toss all-ins/pressure builds pretty easily.
Sometimes the Protoss player is doing something benign like a sentry expand; however, I've found that the extra bunkers do not off set the echo advantage a 1 rax FE provides compared to a sentry expand.
Also worth noting is that if you see 2 gasses being taken very early on (before you scouting scv gets there) its either sentries or dark templar. Make sure your scv survives its original scout and send it back into the main at 6 mins or so. If there are no sentries start an engineering bay asap and then get a turret up in your main and natural. DTs usually hit around 8 minuets so your army should be plenty large to handle them assuming you can see them. Kill them then roll the Protoss player.
Scouting Terran
Overview
Terran is easy to scout. If they FE then they are going for standard macro play. If they are 1 basing then you scan to figure out if its a tank push, banshees, reapers, or hellion drops. If the first rax or OC is late then something is getting proxied.
If 1 basing build a bunker and save scans in case of banshees. Also make sure you have a watch tower. This allows you to either catch the opponents tanks unsieged or make them siege up too far away for a contain.
If the OC or rax is ever late (compare amount done to your own) bunker up and expect something to be proxied.
Zerg:
If a Zerg strays on 1 base bunker the fuck up keep your units spread.
If Zerg stays on 2 bases do the same.
If Zerg takes a 3rd and doesn't saturate do the same.
If Zerg gets a macro hatch off of 2 bases do the same.
Always around 8:30 or so scan of the main to check for mutas. Unexpected mutas are often gg.
Always push out with your first 6 marines to clear watch towers. If you see roaches go marauder heavy or get tanks (I've started getting tanks and my holds have gotten more consistent). People doing FilterSC Zerg build are basically doing a Stephano roach max. 40+ roaches at 12 mins is very hard to hold without tanks.
I find Zerg hardest to scout and find they have the hardest to hold all ins. In other words Zerg is very scary early game. Also always hit a Zerg after an all in even if you hardly have any units. Otherwise they will simply macro out of it and have a just fine economy.
Also don't put bunkers next to other buildings. Leaving a 1 tile space prevents the banes from splashing.
Thoughts on scouting:
It gets a lot more intricate than basic heuristics above. I purposely avoid mentioning things like gas timings and what not. I found in gold and still in plat simply keeping track of when expos are taken is usually all you really need to know.
So for starters simply get good a figuring out if there is early aggression or not. Once you get good at that start trying to figure out what the composition of the early aggression is and when it will hit.
Also when ever anything unexpected happens watch the replay and find out where his build deviated from standard play and use that deviation as a reg flag later when you play. This allows you to recognize anything you have seen before.
Anyways this is a summary of my scouting habits which work very well for me in platinum. I'm sure others here have much more refined scouting practices.