Salutations!
What is the easiest way to recognize foods contaminated with spoilage bacteria?
The easiest way to recognize foods contaminated with spoilage bacteria is when you observe the color change and smell. When the color has got very unprepossessing, and the smell is a bit awful. A bit greenish, blackish on the bread, its straight away a contaminated food with spoilage bacteria leading you to illness.
Hope I helped (:
Have a great day!