I would hazard a guess that this a C-band Receive Only setup by the dish sizes. What you can do will depend on the level of your budget. If you are have a commercial installation that relies upon the performance of these dishes, then you could use the first choice. If you are a hobbyist and have a budget like mine(Abe Lincoln's likeness on a penny can squeal if you pinch him hard enough), you can probably use the second choice.
Choice 1:
You could probably use a spectrum analyzer to check the signal levels of what the LNB(s) can see with the dish in it's current condition. If this is a commercial setup that was performed by a professional installer, check your file cabinets for documentation on the installation. This documentation will usually include some signal performance documentation and most likely a spectrum analyser plot of some sort. If the original installer is still around, check with them for any documentation and see if they can evaluate the current condition of the installation.
Choice 2:
If your setup is a C-Band only setup, the weather should not have much effect on the signal. If your setup is a C/Ku or a Ku only setup, on a nice day find the weakest Ku transponder that you can tune(easier if it is a fixed dish) and record your current signal and quality readings. If you have a receiver that has BER levels, record this also as it is the true Quality. Wait for a very inclement day(shouldn't matter here if it is a c-band setup) and record the same information you previously recorded. Compare the readings to determine if there is measurable deterioration of the signal. You would want to use a Ku transponder here as they are more susceptible to flaws in the reflector's surface and much harder to pull in.
If you know how to use a spectrum analyzer and you know the frequency of the desired signal, you can hook it up and see if your desired signal is at an acceptable level.
There are others that can explain in a lot more detail. For some reason the name Pendragon pops into my head.......