You'll need some pressure gauges to check if you still have refrigerant gas present, unless there is a sight glass on the receiver dryer. Before you get into that though, first check if the compressor's clutch is activating. Lift the hood, start the engine and turn on the A/C to full cold. The belt should already be turning the compressor's pulley and the center of the pulley (which houses the clutch) should also start spinning when the A/C is turned on.
Provided the compressor's clutch is activating on and off, then you probably have a leak. You'd need to regas the system using a gas with a dye in it to find where any leaks are. You'll need to determine which refrigerant is used before you attempt to regas it. Be aware that to regas the system the equipment needed can be costly, depending on the gas used. It may require R-12 refrigerant instead of R-134a, and will either need to be serviced by a mechanic or retrofitted with R-134a connections.