Doesn't seem clogged

Are there any obvious symptoms of a clogged fuel injector? At the previous service of my 2002 VX II Commodore, I was told there was a blockage of a single injector which needed attention. The car has travelled 30,000km and runs and idles smoothly. I disputed the need and was told to check my fuel consumption and return after 3000-4000km for another test. I recently travelled 600km and my fuel consumption was 7.6 litres/100km, which I consider excellent. Normal day-to-day travel uses about 10-11 litres/100km. My underlying concern: is this a legitimate maintenance possibility?