Banned pesticides and fertilizers are banned because they leave traces that are harmful to the human body. Otherwise they generally won’t be banned in the first place. So all they have to do is take random samples and do proper checks. If it was impossible to detect the presence of after effects of such banned substances, there would be no point in banning them since the end product would be no different from normally grown varieties, hence no reason to ban them.

That said, I don’t know how good our processes are, but I do think that more funding needs to be allocated now since the FDA won’t be doing any of their own testing. Turning cargo back at the inspection centers would be an easy way to ban US foods without changing a single law or policy in the country with a high degree of deniability that this was the intent in the first place.