If the compressor clutch is not engaging, I'd would fist start with a static pressure test. You hook up manifold gauges to do this. The system does not have to activate for this test, hence the word "static".
I would ref. the following thread for Larry Bibles static figures.
http://www.peachparts.com/shopforum/showthread.php?t=50725&highlight=static+pressure
If your refrigerant charge is too low, you'll have to hot-wire the circuit at the low pressure switch to draw in additional charge.
If the static test indicates sufficient charge, things get more complicated.