How to document existing network bottlenecks

Hi there,

How to document existing network bottlenecks
I was wondering how many of us has ever got into a project in which you have to document the existing network infrastructure’s bottlenecks. How to start with this kind of documentation? How to be objective? Look at your network from an outsider point of view. What one should document? Are there hard and fast rules to follow. How good is old documentation?
Any help, suggestion, advice and documentation ideas are hearty welcome.

Greetings
Shashi