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Then follow the recipes exactly.

generally 'bad' cans are pretty easy to detect. Bacteria, if there are any alive in the can, will make gasses as they live, grow, and multiply. This will soon ruin the vacuum seal on the lid, causing it to 'pop' outward. If allowed to progress more, the jar could burst.

Generally the principle behind canning is simple. if you elevate the temperature of the can, as well as every single part of it's contents down to the core, to a minimum temperature of 155*F to kill all the bacteria. As long as the lid is sealed and no contamination gets in, the cans will be completely sterile inside. And sterile means sterile. absolutely zero live bacteria as long as the seal remains intact.

Most canning recipes are done at temperatures much higher than 155*F minimum borderline. boiling water is another 50 degrees higher, but the right ammount of time must be used in order to ensure the cool core of the stuff being canned will achieve a safe temperature before you take it out of the heat. Pressure cookers allow a much higher temperature to be achieved, reducing this cook-time dramatically.

The ammount of time a veggie or other food spends at high temperature will dramatically affect it's nutritional value (causes early breakdown), as well as it's cellular integrity (tastes 'flat' or texture is soggy or just plain comes apart), which is why most recipes have been worked to provide the least ammount of cook time while still providing a suitable safety margin. Pressure cookers are best for this generally, and will produce much better, fresher tasting goods (cooked for less time) while still getting them hot enough to be safe.