Kosher without hechsher


If I recall correctly, it is the psak of RYA Shlita that processed foods do not require a hechsher if listed ingredients are deemed kosher. How can one ascertain which listed ingredients are kosher? For example, sparkling water from Aldi contains “natural flavors”. Is that permissible? What are common ingredients that are kosher and what are common problematic ingredients? Thank you

Follow these threads and you will learn.
Natural flavors are fine.