the only way you can know for sure is if you are responsible for every step of the process -
how do you know for sure if the seeds you are planting are non-gmo ?
how do you know that the livestock you are buying have not been given antibiotics or growth hormones ?
how do you know that the ground you're planting those seeds in has not been contaminated ?
you could test everything - but how practical or realistic is that ?
the point is, at some level you have to trust - the question is, how much trust do you have ?