Question : How do I measure or detect network/subnet saturation?

I'm dealing with an issue with a slow webserver performance.  When the server is on subnet A pages load very slow.  If we move this same server to subnet B, it runs great.  The server does receive a lot of traffic.  Are there are tools that we can use to verify our hunch that this is a saturated subnet?  

Answer : How do I measure or detect network/subnet saturation?

I typically run this from a windows based laptop.  The tool itself will sniff any traffic.  Things like apple talk, LPR, Netbios.  It will be able to show you anything that is going on, on a given port on your switch.  If you have cisco switches, I believe you can set all the ports to mirror to a single port and that will show you all the traffic on a given switch.