When co-workers tell you that people are about to get fired, and some changes will be made. I know it's unprofessional of bosses, but sometimes they will tell the people they plan to keep first of their plans to let someone go. But you know people talk, and somehow the information leaks out.
The biggest sign is to watch how your boss tries to treat you. Do they respect you, praise your work, or try to talk down to you, or smile a lot in your face (sneaky)
Usually, it's chemistry you can feel in the air.