Scouting is the key to victory I hear, but can anyone explain how to actually decipher what I've learned? It has to be more than just 'now he can produce X unit'. You usually need some buildings to tech up to other buildings anyways. How can you get a clue to what your opponent is planning just by scouting his buildings?
Rep does not allow for comment, so I'll post as answer. This is NOT meant to be one though. So please, do not upvote it, as it would be a real lack of respect for all who have used the comments to post tips.
Scouting, as said, is vital to understand what your opponent can do.
This does not always translate (specially at higher levels) to what your opponent will do.
So how can you tell what will be done from what you see ?
Short answer is, you can't.
However, you can prepare yourself for what you think could happen, and with enough practice, leave the door open for rapid adaptation.
Now, I am not a high level player, nor close at all, but I have noticed that my game has dramatically improved by watching what others do. How they open, how they pwn me :D, and specially, watching others get defeated (which enables me to go "...hey, he could've done X, or Y,...").
So, the best advice I can give you is play. A lot. Watch your replays, see what your opponent does, see what you do and how you react, watch high level replays, those will teach you about adaptability, watch the Day dailies ("...where we learn to be a better gamer..."), watch the GSL, etc, etc.
Of course, no one would spend 1050 min and 150 gas to build 6 tech barracks to throw you off and then go mass viking. >D
Something that was not said (i think) is that if your oponent is experienced and is scouting your base too you also have to consider what HE saw. So if you have and early gas and then he sees the Spire which you are hiding in the back of your base he will probably assume there will be mutalings soon and might prepare for that. So if you see and armory or engineering bay in his base, you should expect turrets or even Thors, so that factory might not produce siege tanks or helions.
This is just a special example of ZvT, but its similar for any race combination. A building might produce 4 types of units, but the final choice what it will produce is made based on players prefference and what the oponent is building.
Another thing which you should consider is army composition. Some types of units are best used with others (marines with tanks and medivacs, lings with mutas etc.). So if you saw him producing tanks, you might expect air support and heavy infantry (marauders).
The only tip I can give in this area is applicable to most strategy games:
Look for missing buildings you'd expect to see at that point in the game, and extra buildings. No factory but a couple of barracks on Terran would indicate they're going for an early infantry rush, or perhaps with bunkers they're looking to turtle.
If someone's got a particular unit type in mind for an assault you'll see them stop building up their base when they can build that unit, and focus their resources on unit production. Although this is mostly relevant for more inexperienced players.