If you have a set of gauges, you can check the charge. If the low side pressure drops below about 25 psi and no cooling, then you are possibly low on charge.(I said possibly) If the unit is R-12, you can recharge with Hot Shot or R-409a,which works well. The unit may have a sight glass that will show when the unit is fully charged. When there are no more bubbles in the glass, it is charged.