Where did you get your live rock from? How many pounds?
It is in fact possible to have enough cured live rock from startup, that you won't get detectable ammonia levels. I used a SeaChem ammonia test, using the super sensitive version, which takes 30 mins to get an accurate result. I never had more than 0.01ppm of ammonia in my aquarium.
Take a water sample to your LFS
for an ammonia test (which is what I did to confirm my new Seachem kit wasn't bad), or do like Diane suggests and put a couple drops of ammonia in your test vial. It'll probably make it too high to get a proper reading, but you should see a very drastic color change, which'll confirm if the kits still working.