One’s senses of smell and taste to some extent can serve as “chemical detection” (though probably not completely reliably, as some molecules do not “register”). Nevertheless, it seems that chemical reactions taking place over time may often alter the smell/taste perceptual impressions (to the extent remembered).
Even knowing all of the specific chemical contents of a particular flavoring/juice, specific toxicology related answers have not been, and likely cannot easily be, arrived at. In addition to chemical reactions, pathogen growth (may) be another possible concern - making assessments of potential dangers more complicated.
One particular study that I have seen (abstract only; behind pay-wall) that might possibly interest you is:
“Formation of flavorant–propylene Glycol Adducts With Novel Toxicological Properties in Chemically Unstable E-Cigarette Liquids”;
Nicotine & Tobacco Research, Oct 18, 2018.
The supplemental materials (incl. graphical contents appearing within the above article) can be found here.