How to measure local Ethernet speed My Mac mini is connected to my router/modem by Ethernet. I want to know how fast the connection speed is between the Mac and the router.

Obviously, I can use a website like Speedtest.net to get my Internet connection speed, but that doesn't tell me whether the Ethernet connection is a limiting factor, or how much 'spare' capacity there is for local traffic.

The mini serves to other devices. The Ethernet is over a powerline, and the cables are ancient, so may not be optimal.

If I were using Wi-Fi, then the data throughput is shown by holding Alt and clicking on the Wi-Fi menulet icon.
As a rough idea - your speed must be somewhere between 47 and 100 Mbps. Must probably it is 100 Mbps. 

If you want to test, get a second computer and take the connection out of the router and plug it into that. Then you can start a transfer from one to the other and measure the speed. 

One of the most common tools for making such a transfer and measuring its speed is called "iperf". You can install it via HomeBrew (if you have that), or you can download the Mac binary from their webpage.

After installation, start Terminal.app on both computers and run:

iperf -s


on one, and:

iperf -c 1.2.3.4


on the other. Replace 1.2.3.4 with the IP address of the first computer.

After a short while it will tell you the bandwidth in Mbits/sec.