how do i know when a BUC is bad

I don't know anything about troubleshooting Linkstar, however I know that troubleshooting a BUC can be very difficult as there are a number of items in the transmit path that can contribute to the problem (modem, cable, connectors, BUC, feed horn, alignment & polarity). 

Unfortunately the best way to confirm a bad BUC is to replace it with a known good BUC and see if the problem disappears.

Try to eliminate problems in the cable path first.  Measure the voltage on the TX port of the modem, and then measure it again at the end of the cable where it plugs into the BUC.  The two readings should be very close.  If they are not, then the problem is likely in the cable or a connector.  If the readings are the same, then your only real alternative is to replace the BUC with one that you know to be good, and see if that fixes it. 

If the NOC can see your carrier, that would tend to indicate that the BUC is OK (assuming it is generating the proper power); however replacing it is the best way to confirm. 

If the NOC can see your carrier, are they able to perform a cross-pol?  Does the NOC say that they think your BUC is bad?