There's lots of places for an A/C system to leak. Blown 0-ring at any joint, cracked soft or hard hoses/pipes, punctured condenser, cracked evaporator, failed expansion valve, failed pressure switches, leaking front seal on the compressor.
Often, you can simply look for oily stains on the components. Leaks will almost always carry refrigerant system oil out and stain the area around the leak.
After that, as long as there's pressure in the system, a soap bubble solution sprayed or daubed on the hoses, joints etc might show an active leak.
