HI Andy,
There's nothing to test the fiber directly, other than a separate fiber tester device like a light meter or an OTDR
What is usually done is to compare the optical TX and RX levels on both sides, to ensure the signal is within range, if its within range you should be OK. The optical levels are usually found in the show interface transceiver or show controllers interface or show interface detail, you may have to search to find this, don't be confused by the TX and RX thresholds
If for example on the first side the optical levels show TX is 1 and RX is -5 and the facing side has TX 1 and RX -22 then the facing side is RX'ing a lot worse signal than the first side. The loss across in one direction is 6 dBm and the other way is 23 which would indicate a connection problem. Note; a -40 or -50 or -80 means no signal
Whether you will be able to view the optical transmit and receive levels will depend on the optical transceivers that are installed in the equipment. The following document outlines the transceivers that support the DOM capabilities.
http://www.cisco.com/en/US/docs/interfaces_modules/transceiver_modules/compatibility/matrix/OL_8031.html
Without DOM enabled transceivers, the only way to test the fibers is as described by rayframe1; with a light source and power meter.
http://www.exfo.com/Documents/TechDocuments/Specification_Sheets/Fiberbasix-50-angHR.pdf
You can use the TDR feature to diagnose and resolve cabling problems. TDR is supported only on copper Ethernet 10/100/1000 ports. It is not supported on 10/100 ports or small form-factor pluggable (SFP) module ports. For more information about TDR, refer to the software configuration guide for this release.
Regards
Please rate if it helps.
