Was told my evaporator on sub zero going out because ice on back is only couple inches on top coils not throughout entire coil unit. Also may have compressor going out as well. How do I know if I truly need a new compressor?

Would not be compressor. By ice I take that you mean frosting on the evaporator coil. That indicates low freon. Low freon would mean a leak and that would probably be in the evaporator coil.

That indicates that your defrost system is not working. A small amount of ice would indicate a leak, not a lot of ice. You need to check the defrost heater, freezer fan, and defrost timer. Further you should clean both coils because dirt on the coils will unload the evaporator coils causing icing. I would not tamper with the Freon charge at this time as this is a major repair requiring that any leak be found, the unit repaired and vacuumed out. I doubt very much that you have any leak by what you said, but when you clean it look for oil leaks and when you spray the coils with cleaner prior to washing them out look for bubbling. Icing on the coils is commonly mistaken for low Freon, but is actually a symptom of either bad defrost system or unloaded evaporator, although jackleg mechanics will add Freon to an AC to stop icing it will raise the pressure and ruin the compressor. Look here: https://dl.dropbox.com/u/8010163/Sub-Zero_500_series_refrigerator.pdf