There is no way to test diesel fuel injectors without an elaborate test bench. Your dealer or service center may have this equipment. Do not attempt to test the injector by connecting it to the fuel line while out of the engine. You'll bend the line and run the risk of injury. The injector releases fuel under a pressure of 1707 psi (11,761 kPa) or higher; this is more than enough pressure to force atomized fuel through your skin and induce blood poisoning.
Once the injector is set up on a proper test bench, it should be checked for spray pattern, noise, break pressure and leakage. Each one of these characteristics must be within specification or the injector must be replaced.