The easy way is to put your hand on it while someone hits the key.  If
it clicks, the key is sending the solenoid/relay power.  

You said you have have a multimeter or a way to check voltage?

If so, the dark green wire comes from the keyswitch.  When the key goes
to run, this wire should have 12.2+ volts in it.  It is the trigger for
the solenoid.

There are several green wires there, depending on your setup, but only
one will go hot when the key goes to run, it fires up the others...

Then if the relay clicks, you should see power on the cable going to the
starter with the key in run.  If you have power there, then suspect a
bad starter or connection at the starter.  

If no output from the relay, suspect the main cables on the relay dirty
or a dead relay.

You can bypass the starter relay with a booster cable.  The battery
cable goes to one side and the starter cable comes out the other side.
If you put a booster on the starter side and carefully touch it to the
positive battery cable, the starter should spin up.  If the key is
freshly in run it should start then.  (that will get you to the store to
get a new one)

If you do all this and still have no joy, then I would take a booster
cable and run it from the battery negative to the engine block.  That
connection on the main negative cable at the block can go bad too.

When all else fails, you can take a booster cable and put it on the
starter power bolt, then put the other end on the battery positive.
This will spin the starter up and with the key freshly in run it should
fire up.

Hope some of this helps,

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
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