First, feel the air coming from the top of your outdoor unit ( Condenser unit). Feel the temperature of the air coming out. It should be warm to hot. Check the Big copper tube on the outside of the unit.( This is called the Suction line) , Is it cold or sweating? If the air is warm to hot, and the copper tube is cold, and/or sweating, chances are you are ok on your refrigerant charge. You could possibly be low, but not as likely as if the air coming out of the top of the unit was cool or cold and the big tube is warm or not sweating

Is the small (Liquid line ) copper tube too hot to touch and hold your hand to it? If so, you may need to have your condensing unit serviced and cleaned.