As the month progresses, go to Crew Companion --> Options --> Operational. Change your BES and look at the reserve utilization. IT's easy to see how many unused SCs/FSBs are on an individuals Calendar. If you want to dig down a little more, note their file number and look at their CCS calendar and you can see if it was picked up or assigned. Obviously, that won't work if they have their calendar blocked.

The above method won't provide the whole picture, but it will help.