How do I test if it is the injector or computer?

The injector can be tested using a noid light. This is a small light that plugs into the injector connector wire. The light should light every time the injector fires. You can also put a screwdriver onto the injector body and feel or listen for a click or a pulsing sound or vibration. There are scan tools that can test the fuel injectors while the engine is running and perform what is called a leak down injector test if necessary. These scanner tests must be performed by a professional..