1. Your boss tells you. With the exception of one horrible mistake, every time I've fired someone, I've warned them ahead of time that that's what would happen if their performance didn't improve—and sometimes people are shocked anyway. If your boss tells you your job is on the line, she's not kidding.
2. You notice that you're getting a lot more feedback in writing. If your boss used to give you feedback in person and now she's putting critical feedback in E-mails, she may be creating a paper trail in order to build a case for firing you. Many companies require written documentation of problems and warnings before an employee is let go.
3. You feel like your boss is always hassling you about something. If she feels that way too, there's a problem. I've worked with a couple of people who received chronic critical feedback—because their work had chronic serious problems—but somehow, all they focused on was that they found it annoying to be "hassled" so much. They missed the bigger picture and the ultimate point, which was that they were doing a bad job.
4. Your boss is increasingly micromanaging your work. When I become concerned about an employee's performance, I start assuming the problems might go beyond the ones I see on the surface, and I start digging. If your boss is scrutinizing your work far more closely than in the past, there's a reason. It may not be that you're being fired, but something is going on. (And it's OK to ask about it.)
5. Your boss seems noticeably less interested in your opinion. She's not seeking your buy-in and is even outright dismissive about your thoughts on projects. She's lost confidence in you and may even be seeing you as an impediment.
If you're getting signals that you're in danger of being fired, consider taking control of the situation by talking to your boss—and meanwhile, start looking around for other jobs. The worst thing you can do (yet, seemingly, the most common) is to stay in denial.
Alison Green is chief of staff for a medium-size nonprofit where she oversees day-to-day management of the staff as well as hiring, firing, and staff development. She is working with the ManagementCenterto coauthor a book on nonprofit management. Her writings have been published in the Washington Post, the New York Times, Maxim, and dozens of other newspapers. She blogs at Ask a Manager.