Piotr, from cli issue " show interface "

if you are particularly looking for specific interface issue "show interface fastethernet0/#" where # is the port number.

the "show interface fastethernet0/3"

will show interface stats for port 3, look for input errors, CRC, Runts, Giants statistics as these are indicators of communications errors between the port and the connected system. Same principle applies for gigabit interfaces if you have any uplinks. " show interface gigabitethernet0/#.

HTH

Jorge