Working blind, but not for long!
At the moment every stage that goes up is lost; if it works great. If not then the hope is that details analysis of the telemetry and design and provide a definitive cause, but I imagine that often leaves gaps in their knowledge.
However what will soon be revolutionary (beyond simple reuse/money saving of course) for SpaceX and the Falcon is that they will be getting their first stage BACK. That means that can then tear the thing down and see what held up well, what broke (but did not result in a critical failure) and what was stressed near it's limits. Once they have that I image that rocket reliability will jump well ahead of anything else.
Also keep in mind much of the Falcon 9 Stage 1 hardware/tech is present (or will be) in their other stages, so lessons learnt there can be applied elsewhere.