Hi,
do a "sh fex" this will tell you show you the fex number, description, model number, etc...
now lets say the first fex is fex 101. Now do a "sh fex 101" this should show you the physical interface that are associated with that fex and are up and running, see example
Fabric interface state:
Po101 - Interface Up. State: Active
Eth1/5 - Interface Up. State: Active
Eth3/5 - Interface Up. State: Active
now do a sh int e1/5 or 3/5 to see how much traffic going through that interface.
HTH