If you mean an intake leak at the injector, the classic way to detect it is to spray flammable solvent directly at the injector and listen for an idle speed increase. If you mean a poorly functioning injector, I would disconnect the harness plugs at each injector looking for a worse idle, meaning that particular injector was doing its job. The one making no difference would be the suspect. Of course, you might find a loose or dirty electrical connection this way.