How can tuna/bonito be tested for histamine?
If you say that your handling of the fish was a 5 minute walk and to cut it up and freeze it, it is more than likely that the contamination occurred during the supply chain. Once the enzyme histidine decarboxylase has been formed, it can continue to produce histamine in the fish even if the bacteria are not active.

There are test kits available for checking histamine levels but they are intended for commercial use. http://www.noackgroup.com/Live/ProductCatalog_en.YoCms?GROUP=%24D_FAA_07_01#_1NEO9505 is one of the places that has them.