The best way to know if the food in a thermos has gone bad is to monitor its smell, the time it has been in the flask, and the temperature changes. First, it is imperative to trust your sense of smell and check for any rancid, sour, or strange smell coming from the flask. If it doesn’t smell like it should, then throw it away—better to be safe than sorry.  Likewise, it’s essential to keep track of the time the food has been stored in the thermos—anything longer than the time recommended by the manufacturer should require careful monitoring. Granted, having food in the thermos for 6 hours instead of the 12 specified in the instruction manual is not a fool-proof guarantee of it being rotten. Still, most food is temperature-sensitive, and often contains many different ingredients with different storage and temperature recommendations, so always best to check it twice before eating. Finally, if you’re carrying temperature-sensitive food or want to be extra safe, you can always take a small food thermometer with you—that way, you can always tell if the food in your thermos has remained within the safe temperature above 140°F (60°C).