-EDIT-
A transistor can test good but fail under load.
Do you have a multi-meter?
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OK, that's a dumb question. Of course you do to perform the test you mention.
SO, connect meter black lead to ground and put red lead on the connector pin feeding ground (on signal) down to that solenoid. In solenoid test, you should see solenoid B+ voltage until it is supposed to fire, then the value will pulse low for a half second. If you need to, verify the test by probing a working solenoid to see the behavior you should be seeing.