You can test a switch diode "in circuit" the same way you test a loose diode.
Meter on diode check.
Game power OFF
black and red leads on opposite ends.
One way the reading will be "open" or nil, the other way will be about .5 to .7
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The Place to go for Pinball Repair Info

Bad switch diodes are only one potential cause of phantom switch activation. Diode testing for both microswitch diodes and leaf switch diodes can be found below as well. Read the following headings:

Phantom Switch Closures: a Shorted Switch.
Bad Switch Diode.
Fail-Safe Diode Test.
Testing a Microswitch's Diode, without removal.
Testing a Blade/Leaf Switch's Diode.
http://techniek.flipperwinkel.nl/wms11/index3.htm

I'd first go into the switch test and start playing around with the different switches you think are having issues. The display should indicate what switches are being activated. If, for instance, you press on the lock switch several times and briefly it indicates it was a drop target, then write both those switches down.

Look at the switch matrix to determine if they are on the same row or column. Once you can see a pattern of which switches are causing problems, look at each switch on that row or column you suspect. Check for a good mechanical connection with the diodes and the switch lugs as well as the wiring to the switches. Make sure a switch lug isn't touching something metal. Also, if one of the switches happens to be an opto, a bad or shorted opto can cause issues too.

If the switch can be isolated by disconnecting it try that and see if the problem goes away. Yes a diode can be tested with a DMM set to the diode setting. You should get between .4 and .6VDC with the leads one way and infinite with the leads the other. Test a few to compare and see if one is way out of spec from any you may suspect may be bad.