They really smell bad. If you have any rotten eggs in your incubator you can usually smell them before they explode. If I am smelling a strange smell coming from my incubator I smell each egg. You can put your nose down there to smell them each or pick them up and smell them.
Also if you are candling your eggs if you have a egg that is totally dark be careful how you are handling that egg.
Remove any eggs you suspect of being rotten carefully and immediately