Here's a link to a thread on testing the CTS (coolant temp. switch) (sorry, don't know how to do shorter URLs):
http://www.peachparts.com/shopforum/showthread.php?threadid=16757&highlight=testing+cts
I did a search on "testing pressure switch" and got some useful info from Arthur Dalton as well but did not see a reference to
specific pressure/resistance value. The collected wisdom was that low refrigerant charge was much more likely than a bad switch, and the first order of business seems to be to make sure the AC system is adequately charged before condemning the switch.