Here are the general rules put forward by the government: Between 40°F and 140°F (5°C and 60°C) bacteria multiply rapidly, so food should not linger in that temperature range. It should either be in the refrigerator, freezer, or oven. It should not be out for more than two hours (1 hour if the room temperature is above 90°F). Don't keep food if it's been standing out for more than two hours. Don't taste test it, either. Date leftovers so they can be used within a safe time. Generally, they remain safe when refrigerated for three to five days. If in doubt, throw it out. It’s best if you store leftovers in a plastic container that can be sealed tightly.
Reheat foods thoroughly to 165°F (75°C), or until hot and steaming. Bring gravy or sauce to a rolling boil. Place carved meat or poultry in a casserole. If desired, sprinkle with broth to keep it moist. Cover the dish and reheat in an oven set no lower than 325°F (160°C) or a microwave oven.
Beyond that, use your common sense and do what cooks have done for centuries — how does the food smell and how does it look? You can’t always see or smell that the food is good, but you can generally tell pretty quickly when it’s outlived its usefulness.