One of the more common things we see are air conditioning systems that have been overcharged with refrigerant. This common rip off is usually done when you call for a simple maintenance service and they are adding refrigerant to make money off you. I suspect that many times the homeowner complains that it just is not getting cold enough and many times this is caused by other issues such as dirty air filters, dirty outdoor condenser coil, dirty indoor blower and dirty indoor evaporator coils.
After these basic measures are taken then the system can be evaluated for proper refrigerant levels. While it is difficult to provide you with what pressure readings you should see because each system and every outdoor air and indoor air condition/temperature has an effect on the readings as well as the fact that there are two common refrigerants in the market, R22 and R410a. The question to ask a technician is to document right on the work order what the pressure readings are and ask to see the gauges when they write this down, this calls them to the table and makes them be honest as you can easily see if incorrect numbers are written down and these readings can be checked by other technicians and companies. Make sure they also write down the numbers after they add refrigerant, and stick around and watch them. No smart technician trying to rip you off is going to document the rip off in action.