Turn the A/C to maximum cold and put the system in "recirculate" mode (assuming there is one) - it's usually a button with an arrow bent back on itself. Within a minute or two the air from the vents should be noticeably chilly. If you have a digital thermometer at home - perhaps in the kitchen then bring that along and you should be able to measure the temperature of the air coming from the vents. The optimum temperature should be around 6° C.
We recommend you use the air conditioning all year round regardless the temperature your heater is set to. AC is not just for cooling - in the Winter it also removes moisture from the air so prevents misting of windows. Also modern cars are very efficient and you should not notice any increase in fuel consumption. If you prefer not to have the AC switched on constantly then you should run it for at least 10 minutes once per week. This will ensure the refrigerant and oil circulates and prevents the seals from drying out which can cause them to leak.