I assume that you have all the plant documentation which contains the operating data and curves for the major components that you mention.
If you do, check the cooling tower curves for the CW temperature that is to be expected at the ambient in question. If the CW is not as predicted, do the necessary cooling tower checks to get the tower in spec (or determine that it is not performing).
Next, with a known CW temperature, check your condenser curves for its performance at those conditions. The CW temperature rise is important, as is the approach temperature (condensate saturated temperature vs. CW outlet temperature.) These all give you clues as to the condenser's operation and condition and then give you a predicted back pressure for the CW conditions.
Then use your turbine heat balances to determine what your predicted performance should be with the back pressure that you have regardless if it is on spec of off spec due to the cooling tower or condenser variations. (It could be better.)
If you are an engineer, it should be a fun trip if you remember your thermo (most of the electricals that I ever knew hated thermo). If you have ME's or CHE's on site, ask for their help. If you aren't an engineer don't start because it will be way over your head.
rmw