Whenever I get a chance I always capture broadcast and multicast packets.
I figure out a great deal about the network and host configurations as well as possible tuning recommendations. When I show clients what I find by just capturing ‘idle’ traffic they are typically amazed and ask how I figured it out.
The first step is capture some packets and as you get some experience, the rest becomes easy.
In this video I zero in on IPX but you can use the same methodology for any protocol or device.