One way of doing this is to select products that are Fertmark registered. Here is an explanation of the Fertmark programme on the NZ Fertiliser Association website:
The Fertmark programme was established in 1992 to give New Zealand farmers confidence in the quality of fertilisers and associated advertising.
It now includes the requirements of the Agricultural Compounds and Veterinary Medicines Act that fertilisers are supplied in such a way as to minimise risks to food safety and animal welfare.
The Fertmark Code of Practice relates to all fertilisers sold under the Fertmark programme.
Farmers can be confident that fertilisers supplied from Fertmark registered companies meet the claimed nutrient content and, if used properly, do not pose hazards to food safety or animal welfare.
Fertmark provides quality assurance on claimed nutrient content to farmers purchasing fertiliser, with Fertmark certified products displaying a green tick.
To ensure ongoing product consistency, regular product samples are analysed, and procedures and results are subject to external, independent audit.