When I put this mechanic's stethoscope on and probe the suspected fuel injector by placing the tip of the tool against the injector's body. Heard no clicking of injector's valve.
Any idea to test injector?
Or I should get one injector tester?
but all I found is the common rail injector tester

 

Found several injector tester designs on the web. This one looked interesting:
http://www.dinofab.com/fuel_injector_tester.html