The easy path, as someone suggested, is a kill-a-watt that you just plug it into.
The less intrusive path for some definition of less intrusive is a clamp ammeter if you can expose one of the AC wires (you have to clamp around an individual wire, not both hot and neutral). But then you don't need to unplug the system to measure it.
The third overkill option is to have it plugged into a full-time power monitoring and control device, such as a zigbee home automation plug switch. ;)
This can actually be life threatening if done without proper tools and working fast fuses. So just use a kill-a-watt or any other such tool for safety! NB they're maybe not 100% exact but good enough to give an estimate.
Clamp meters are completely safe. The only risk is if you DIY your power cable so you can clamp one lead, but that's not necessary if you just buy an AC line splitter plug. And those often come with the hot looped around so you can get a 10x reading for better fidelity at lower current draw.
But these days I just skip the clamp meter and throw Ikea Inspelning zigbee plugs anywhere I want power measurement.
The less intrusive path for some definition of less intrusive is a clamp ammeter if you can expose one of the AC wires (you have to clamp around an individual wire, not both hot and neutral). But then you don't need to unplug the system to measure it.
The third overkill option is to have it plugged into a full-time power monitoring and control device, such as a zigbee home automation plug switch. ;)