People may die due to food poisoning. What are the major dangers? Sometimes in restaurants people eat a meal and get sick. How can we see that the food is dangerous? 
As said in the other answer, there are many types of food poisoning, some of which can be detected, some of which can't. The best bet is to trust your senses. If you have some experience and something doesn't "taste right" it's probably good to not eat it. If you're preparing your own food go for smell and look first (strange or rotten smell, discolorations, mold, ...). You can also taste a bit in most cases, where you're not sure and spit it out in case you found it spoiled.  One form of food poisoning i want to mention specifically, because, although rather rare, it is especially nasty and dangerous: botulism. It is caused by a certain kind of bacteria that can live on nearly all our food, but are harmless by itself and are destroyed by our stomach acid. When they are kept in a low-oxygen environment however, as in tinned food that is preserved in oil, they multiply and produce a very toxic poison that can lead to everything from tiredness to diarrhea and, if not treated, even death. It is rather rare, though, so don't be too worried. The poison is tasteless, however, so it is very hard to detect. It can for example be found in tinned food that is not properly preserved. If you want to preserve food always be sure to cook it long enough and add enough acid to kill all bacteria beforehand. The preserving community on the internet is of much help here.   On a side note: these bacteria are also the main reason why parents are often advised to not give raw veggies and honey to their babies, as their stomach acid is not strong enough to kill them.  