What are my options for monitoring utilization of an interface with a higher degree of resolution?

tl;dr - How can I tell if my WAN is regularly getting saturated?

We've got a 100/100 connection from one of our ISPs. Recently I've been getting reports and behavior that makes me suspect it's getting overly congested. But when I look at my SolarWinds graph of the interface, it shows it's only using (on average) 10mbps utilized with occasional spikes to 30mbps.

I understand the way that ethernet works, that being a serial interface it only runs at one speed and the reports from SWinds are simply a reading of utilization over a set period of time. Is there a way to get higher resolution reports of an interface's bandwidth usage?