Yes, WireShark will provide you with an overview of your network traffic; try analyzing traffic from multiple points (I recommend using it on your gateway server if it's a Windows box) for solid, more accurate results.
Porkchopexpress *giggle* is correct about the DHCP issue, but do you know what exactly you're looking for besides high network utilization? Finding out what type of traffic is excessive can be determined by your environment, how many users are within your network, legacy equipment, and a few other factors but you get the point.
Here are a few questions to help point you in the right direction:
Are you seeing excessive ARP requests? Any workstations looking for another subnet that's not attached to your network, or once was? Unknown traffic to outside \ inside IP addresses?
One last thing would be to provide us with an overview of your network including how many users are present, and the number of servers and workstations.
Cheers.