If something just smells like rotting bad food your okay. Anaerobic is a smell you probably haven't smelled until you have a bin that's gone anaerobic. Usually that smell will be at the bottom of the bin where there is no air and it's too wet. When you get a whiff of it you will know, it will send a panic into your worming being.
I wish I could send a smell electronically. But if the food just smells like it's rotting you're okay, especially if it is in the top layers where it can get air.
Food that is smelly and molding is starting to decompose. That's when the worms will really go after it. Just make sure and bury it with a layer of bedding.
Most importantly, as I always say, take your cues from your worms. If they appear to be thriving and happy you're okay. But you need to get your hands and nose into the bin. Don't just go by what you see on the top layer. Get to the bottom of things as they say, at least once a week, especially until you really feel comfortable with what you're doing. Don't worry; the worms will do fine with that. The will appreciate the air you are incorporating into the worm bin, especially if you have an anaerobic situation developing. Air is the best fix for anaerobic.