Try (as root) the command "ethtool <interface name>"
The report generated will show (around the middle) the current speed, and the duplex mode. If you see "Duplex: Half" then you are operating at a reduced speed. You should also (just above the "Duplex" entry) see the speed the device recognizes.
ethtool can also set parameters, but I've never needed to change the defaults.
What I get for my internet connection is limited by the router:
# ethtool p6p1
Settings for p6p1:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
Supported pause frame use: No
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
Advertised pause frame use: No
Advertised auto-negotiation: Yes
Port: Twisted Pair
Supports Wake-on: pumbg
Current message level: 0x00000001 (1)
Link detected: yes