Whenever I need to look at network traffic to see what is actually going on, I use Wireshark.
There are other options however, but I prefer the “true” nature of Wireshark. What you see is what you get.
As Laura Chappell puts it: “Packets don’t lie!”
Tools like Microsoft’s Network Monitor might give a more insightful look at the total picture of what is going on, but it will not show what truly flies by over the network – it will show an excerpt of the network traffic mixed with all kinds of application/OS layer data.
Fine for those who want that, but I focus on the network layer
solely – believe me there is more than enough to look at there!