I thought botulism was unsmellable*? Or am I just looking for general rancidness? Anyway, DH bought me some lovely canning equipment for Christmas and I am eager to try my hand at some preserves with the late spring/summer fruits!
There should be no question but that the food is spoiled, but a few people manage to ignore it for some reason. The most important thing to do before you do much canning is to thoroughly read the best canning book you can find. I have a tattered old copy of the "Ball Canning" book, and recently bought a new one! I read the directions every single time, and follow the recipes exactly. I actually weigh the fruit/vegetables and the sugar or other ingredients called for. An accurate timer and good, clean canning jars with new seals is also important.
Before you use them, feel the top edge of each jar and discard any that feel rough or have the slightest chip. Look at them with a magnifying glass if you need to. Those jars need to be marked with something permanent if you plan to use them to store dry goods or for refrigerator storage of leftovers, but don't ever use them for canning. I've got a whole box of them, bottoms spray painted RED.
I canned some meat exactly once and didn't like it, so never tried it again. It can be done, and done safely, but you need a pressure canner and you must follow the directions religiously.