How do you know when a cultured item is no longer safe to consume?

Cultured milk products rarely become unsafe.

Yogurt in particular is so acidic and teeming with bacteria already that it can't really go bad per se. It will get moldy as others have said.

I culture my own buttermilk, yogurt, kefir, cheese, etc. Many times they stay on the counter for days at a time. They get more and more sour (if they are the kind that incubate at room temperature) but they can't really go bad. Even sour cream doesn't become dangerous- the mold just tastes bad.

To second what kajaco said: if I do have cultured dairy get too sour or moldy I will wipe off the mold and bake something with it.

It should be noted that non-cultured milk is completely different. Of course it goes bad fast but after even a short time it is too bad to bake with. And never ever EVER use old milk to culture your own yogurt or cheese. The bad bacteria will overwhelm the good and you will end up with liquid that will haunt you in your nightmares!

-- Edit --

https://web.archive.org/web/20161129153701/http://nchfp.uga.edu:80/publications/nchfp/factsheets/yogurt.html (OR pdf)

I did some research and it seems that they can eventually go bad. It doesn't fit my experience but I'll defer to the scientists. Trust your nose.

Before something is unsafe to eat, it usually becomes unappetizing or unappealing. This is observed more by how it looks and smells than how it tastes (why would you taste it if it looks or smells bad?). Unless your choice is to eat something unappetizing or to go hungry, most people would not use it but throw it out if it is unappetizing.

There are exceptions, however. I would not drink sour milk, for instance, but I will bake with it (this requires adjusting or changing ingredients such as baking powder vs. baking soda, but that's another question).

"When in doubt, throw it out..." (Or compost it, etc.)

Seriously, all it takes is one good bout of food poisoning and you form a habit of not eating anything suspicious. I will still cut off a moldy end of hard cheese and eat the other part, but if milk or yogurt seems at all suspicious, I get rid of it.

Personally, I sniff and if it smells right I'll generally take the risk of using it. The hygiene department may not agree though. I really trust my sense of smell. On the other hand if I see an ugly mold growing, I may change my mind.

FWIW - It's usually yogurt and a few cheeses where I run into this issue...

It is not safe to eat anything with mold on it ! See, some of the spores are microscopic so it is not visible to your eye, I would dump it. Mold is a mycotoxin it can cause biotoxin illness,. This is what I do I put a sticky on the lid of my homemade yogurt and I just keep it for two weeks max that is it.