Canada

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.

I'm not sure how accurate this is, but from https://ottawacitizen.com/news/local-news/can-canada-still-trust-u-s-food-safety

However, this recognition does not exempt food imports from meeting the regulatory requirements. All food sold in Canada, whether it is domestic or imported, must meet Canadian food safety requirements.”
Canada has a robust system in place through onsite inspections and verifications to verify imported products comply with Canadian regulations, it said, adding that the CFIA is tracking any changes in the U.S. and other exporting countries “and will take any necessary action to continue to protect the health of Canadians and maintain a safe food supply.”

The implication being that they can inspect produce already certified safe by the FDA and that they will do so if they consider it necessary.

Whether or not they're already determined that's the case (vs still thinking about it), well....

It's bigger, as it's not just food safety at risk, but medication and medical device safety is at risk too, as explained in https://www.ctvnews.ca/health/article/amid-deep-cuts-to-the-us-fda-experts-warn-canadians-could-lose-vital-safety-information/