I'm still going to say that you should get a test kit yourself...not just because of this, but because in an emergency situation you're not going to want to have to rely on getting to a store and have them test it...
Unless your tank somehow is outside the rules of basic chemistry, then I see no other way (barring the low, low, low chance you have denitrification) you could have 0 ppm NO3 in a fully stocked, established aquarium.