It's not just that spoiled food starts to show symptoms, we were naturally selected for aeons to detect these symptoms.
Telling the difference between food and poison is one of the most important survival skills for any higher organism. Any hairless monkeys which couldn't tell the difference and ate spoiled food died and didn't pass on their genes. Those who were able to tell the difference survived and passed on their genes to us.
This is how humans have evolved the ability to smell if food is good or bad. When microbes do their work, they produce poisonous chemicals. Human olfactory senses evolved to detect these chemicals and respond to them with a strong aversion. This is why bad food "stinks". Similarly we evolved an instinct to notice certain colors or textures which hint at spoilage and find them distasteful.
So if it looks spoiled, smells spoiled or tastes spoiled, follow your instincts and don't eat it.
But remember that your natural senses to detect spoiled food are not perfect. In an early phase of spoilage, the signs might not be there yet, but the food is already bad. There are certain kinds of spoilage you can not detect instinctively at all. You can only protect yourself from them by following common food safety rules. There are also some kinds of foods which appear spoiled but are actually safe to eat (like blue cheese. Throw it away when you see mold on it which is not blue).