The world is full of fairly nice people who will spill anything you said to them to anyone for any reason in the workplace. And I'm not cynical about people, I just think it's the truth.

There are no warning signs. Just pretend everything you say to a coworker is on a public ledger.

In some cases the true, actual friends you make in the workplace, the ones you'd still hang out with years after your employment, ca be trusted. But it's still probably better to not tell them anything that could make you look seriously bad either.