Unfortunately, no Star Trek style tricorder exists for analyzing food nutrient density. You'd essentially need a chemistry lab so you could extract and measure all the nutrients.
Currently, Brix is the best concept for estimating nutrient density in food. You can use hand held refractometers to measure brix (
should cost about $100, or less). There are folks working on comparaison charts for common food crops, and their brix ratings, but these are still in the works (as far as I know). Perhaps you could show your customers the difference between your food and low quality store bought food.
If you google nutrient density and brix, you should find lots of info on this.
Here's one web page (selling refractometers) with a brief overview:
http://www.naturalcheck.com/brix_nutrient_testing.php