Speed of network connection?

I have dietpi on an Odroid C2 and I would like to know at what speed the network adapter is connected to my switch (1Gb oder 100Mb). I could not find this in one of the dietpi config menus. Is there a command that gives me this info from the command line?
Thanks a lot

ethtool eth0 should include the connection type/speed.

Thanks, worked like a breeze.