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: http://www.youtube.com/watch?v=085PcLgg3Q4&NR=1
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.