Yeah, the best way to scout is to try to keep your initial probe alive for as long as possible. Unfortunately right after your probe dies and your observer gets to your opponent's base there is a crucial time where you don't know what your opponent is doing above their ramp. It's unlikely that your proxy assimilator will be in a good position to see your opponent's tech.
I feel like this is a huge flaw to the game as protoss just have to play blindly. However it's not much better for zerg, they can only suicide an overlord in this early in the game. The overlord dies relatively quickly to marines too so it can be easy to hide one's tech such as in this game between Idra and Silver:
Terrans have the option of using reapers to jump through cliffs, using floating buildings which are either immune to the other races basic unit or have enough life to fly through them, or using a scan, which although it isn't a surefire way of seeing your opponent's tech, it does have a large radius and therefore likely can spot it.
Sucks not knowing whether your opponent is teching or has a mass marine ball :/
One thing protoss can do to even things out though, is to build a pylon in a hidden location far away from both players bases and to hide their tech there.
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[9] 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
Cheers!
Best Answer
Certain units are better at scouting than others, but you should always scout constantly. There are a couple units which work well for scouting:
Goals of Scouting:
When to Scout:
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.