Check for link lights at both the station and the hub or switch end.
However, due to increased software control the presence of a link light is not
a guarantee that the port works. The absence of a link light is still a
reliable indication of a problem.
Disconnect the problem station or the
station identified as the source of errored frames from the network. Attach a
monitoring tool in its place and use the original cable from the problem
station.
Also, test from the network side back toward the suspect
station. Use a spare cable to attach the test tool to the station and monitor
the power-up process. Observe the link process and the protocols that the
station is sending. Ensure that the station is communicating, as many problems
relate to station configurations not recognizing or attempting to utilize the
network adapter. Look for signal strength and other physical layer parameters.
Not all network adapter faults will be exhibited during testing. If the
fault appears to be intermittent it may be appropriate to replace the network
adapter and driver software as part of the diagnostic process. If that solves
the problem then try exchanging just the network adapter to isolate the problem
to the network adapter or the driver software.

Some
portable PC network adapter cards ship with a special power saving feature
enabled. This feature causes the NIC to listen for link pulse, but it will not
transmit anything at all (including link pulse) until it hears a signal on the
receive circuit. To conserve power the entire transmit circuit in the NIC is
shut down until the receive circuit indicates that a transmission is warranted.
This feature has been known to cause link problems with some other network
devices. Try disabling this feature in the software configuration of the NIC,
or use a test tool to see what signals are offered by the NIC.