hi, HADR.
First, is to check the implementation of management of purchased material which means that:
1. Source from reputable supplier (which means that they apply good agricultural practices)
2. Audit the system of that supplier
3. Establish confidence level
4. Have what you require in specification
5. Monitor the performance
Depending on the above you may need to check for HM and Pesticide for every delivery, however this may not be sustainable and you also need the risk that you will be taking (as this will rely on sampling). Should you able to establish /manage by PRP, then you may opt to have less frequency of test (due diligence)