First you need to burn the food product for a while to make sure that most of what is left is heavy metals in the ashes. (you typically need a flask for that, a

First you need to burn the food product for a while to make sure that most of what is left is heavy metals in the ashes. (you typically need a flask for that, and an apparatus to get all the fumes from the burning process safely out). then from the ashes you need to dissolve them in water (easier said that done), and use reagents based on the type of metal you want to detect from the sample (titration). If you measure accurately the volume of water used to dissolve and the amount of ashes coming from the food product, you can determine a concentration for a specific heavy metal. Sigma-Aldritch probably sells the reagents you need for heavy metal testing.