the fan should always come on when you switch to A/C. if it does not switch on, then the resistor on the fan is blown. they aren't cheap either.
your best bet is to check the temp of the rad at 3/4 temp. if it feels cool on the driver's side, then your t-stat is crap, or you have air/crap in the rad which needs to be bled/cleaned. (remember that the thermoswitch that controls the fan is on the driver's side...)
to check and see if your fan is working from the input of the thermoswitch, disconnect the harness going to it (lower portion of the rad) and jump two of the connections with a wire. try different combinations, and you should get high and low speeds. if you're getting both high and low, just clean the contacts on the harness and maybe the thermoswitch, and re-install it. just cleaning those contacts might fix it for you.
i was unfortunate enough to get a faulty thermostat... at 100C it opened a little bit, but not all the way... the faster i drove, the hotter the car got which was puzzling me!