Hi Rod,
There is no need to replace all of them...
The easiest way to test the transistor is with Ohm-meter.. and transistor outside the circuit (but it should work with transistor soldered in place as well). However, the multimeter should have high enough test voltage, to deal with Darlingtons. Try using Mohm range if diode test doesn't work.
You should have low resistance from Base to Emitter, and from Base to Collector (Tab).
However, in opposite direction you should have high resistance.
From Collector to Emitter you should have high resistance, while in the opposite direction, you should have low (because of build-in protection diode).
Try to experiment with the TIP122 you have, not mounted.
The data sheet is here:
http://www.fairchildsemi.com/ds/TI%2FTIP122.pdf
If you have problems, give me a ring or PM me.
Or you just replace all of them.
Later, you can separate survivors from casualties :-) ..