That's how iperf3.x works.
The "client" wants to establish a control channel to the "server" before any UDP transfer is started. That's why you can observe some TCP before the UDP transfer starts. I haven't checked, but I believe that there is also a separate control channel connection when running TCP tests.
In contrast, iPerf 1.7 and 2....
usually the NIC and switch ports have LEDs that light up or you could check the port status
a connected switch should show the device's MAC address on the port if it's sent just a single frame
run a packet capture on the connected port, usually there's some traffic, or the device even tries to identify itself via LLDP
check the DHCP server if there's a lease ...