Your best bet is to look at the "Interface Details" page in NPM. There are two charts that will shine some light on bandwidth utilization:
Min/Max/Average bps In/Out
You can also setup an alert based on bandwidth or percent utilization to send out an email when the threshold is met.
as a rule of thumb, what percent utilization is considered good, boardline and a problem?
Also, what is the rule of thumb for Min/Max/Average bps In/Out?
We normally use 75% as the benchmark for high utilization. I normally use the Min/MAX/Avg RX+TX chart to look at my utilization pattern. Netflow digs deeper by telling you who is using the bandwidth and how much so that you can for instance do some capacity planning or redefine your network access rules.
The standard rule of thumb I have always gone by is that you don't want to consistently exceed your WAN links by 70% and your LAN links by 30%.
That being said when you are looking at the charts and graphs in Orion these are based off of a 24 hour clock by default. So if you are looking at the monthly utilization of a circuit for a location that is 8-5 M-F your results will likely be skewed and you may want to adjust for their actual working times.
There are a couple of examples on how to do that but here is one for creating a report based on business hours.
BW utilization can be a useless measure if you use Quality of Service policies on your links. We have to go be user experience. Our links can be maxed out and we never hear from the users.
It just depends how much detail you need to identify the bandwidth issue. NPM which has being mentioned already is good at using SNMP to read values from your network devices. One of the most commonly used values is bps of an interface. You can use this to create alerts when bandwidth goes over certain levels. Other may correct me on this but I find that SNMP data does not have enough information (sometimes) for troubleshooting.
Your next option could be to use NetFlow or one of the other flow standards (jflow, ipfix, sflow). Typically NetFlow is available on a L3 device like a router or switch which has a layer 3 feature set. Once enabled you end up information like what is making connections on your network and how much bandwidth these conversations use up. The NTA product can also graph this data so you can see what utilisation is like amongst other things.
If want to look at what is happening on your LAN and you don’t have NetFlow features available on your network switches then you could look at using a SPAN port. This allows you to take a direct copy of the traffic as it moves through the switch. You could use something like nProbe to convert this raw traffic to flow which can then be processed by NTA. More info at Orion NTA and nProbe: Analyzing bandwidth hogs without flow-capable network equipment
Finally, we’ve integrated our LANGuardian traffic analysis product with NPM so when monitoring and troubleshooting you get a very comprehensive picture and all the detail you need via one dashboard. An online demo of it is available here