1990 240SX Won't Start

I need your expertise. The car had been running fine, but now it will not 
start turning the key, but will start with a push and popping the clutch. 
Thought it most likely was the starter, so I pulled it and had it tested. 
Of course it and the solenoid tested out just fine.  Back in it went.

There is correct voltage to the starter, the solenoid and the battery 
voltage is fine. Plugs, wires and distributor cap are also okay.  I didn't

test the starter relay (wasn't quite sure how).  I realize the trouble
can't 
be diagnosed on-line, but I'm open to suggestions as to what to try/test 
next.  I've got a Chilton manual for it and am technically inclined to a 
point, but as I've gotten older, the point has gotten duller.  Thanks in 
advance for the suggestions and input.