How much of a scenario do you ought to know (when you start playing it)?

[Mowgli]

Having absolved my first couple of PBEMs, there is one thing I keep wondering. Maybe the veterans can give me their opinion on this. 

My question is: How much am I supposed to know about a scenario when/before I play it? In particular, I'm refering to the enemy units' initial dispositions and composition, supply/command levels (perhaps even locations of supply sources), reinforcements, nuclear/chemical weapon allowances, etc. etc. All the stuff you can see when you load a fresh scenario from the opposing side's perspective. Obviously the things that can be found in the parameter data are common knowledge for both sides, but what about this other stuff?

It is obvious that knowledge or the lack thereof can make a huge difference. For example, in one of my PBEMs, I didn't know whether the opponent had a supply source on the near side of the river, so I cautiously assumed he had and therefore didn't dare to defend against his crossing. 

It is also an important question for balancing (in particular: time limits). I sometimes get the impression that some scenarios might be balanced based on the assumption that both players know the starting positions and capabilities of the opposing side but of course I could be wrong. 

How do you handle this? Do you try to go into your games totally blind? Do you expect it of your opponent? Unfortunately, the scenario-descriptions never give players any instructions on this. And it's also just natural that the evil (?) temptation to take a look is always there. And I've even found myself taking looks without any evil afterthoughts at all - when I had to pick a scenario and I needed to check whether a scenario had a suitable size and would be fun to play for both sides (e.g.checking the number of "fixed" units...). It's hard to "unsee" things.