Для Windows: Get-NetAdapter Ethernet | fl Name, InterfaceDescription, ifIndex, Status, MacAddress, LinkSpeed
Для старых версий Windows: wmic NIC where "NetEnabled='true'" get "Name,Speed,MACAddress"
Для linux: cat /sys/class/net/eth0/speed
Для старых версий Windows: wmic NIC where "NetEnabled='true'" get "Name,Speed,MACAddress"
Для linux: cat /sys/class/net/eth0/speed