Your Handbook: Your employee handbook may well have some policies where your employer tells you they're spying. For instance, there may be a policy saying that your work on your computer belongs to the company, that they monitor emails sent and received on company devices, that they record calls for customer service purposes, or even that they monitor your social media. If it says so in your handbook, that's a good indication that big employer is watching.
Contract: This is an easy one. If you have a contract with clauses saying your computer and company-owned devices aren't private, they probably aren't. If your contract says your phone calls are being monitored, they probably are.
Other policies: Sometimes, the policies indicating the company is watching you are separate from the handbook. Read those new policies your employer sends out. You might be surprised what they say. If you have to sign policies saying you've received them, you'd better read and understand them. You might sign something saying you agree that your conversations can be taped, allowing your photos to be used in company advertising, or acknowledging that you have no expectation of privacy at work (which you probably don't).
Cameras: If you see cameras around the workplace, someone is either behind them or there's a device recording your actions. While there are restrictions on recording audio, most employers are allowed to videotape you at work. Cameras are easy to hide, so just because you can't see them doesn't mean they aren't there.
The computer camera is on: If you see your computer's camera light on, someone may be watching you up close and personal. I've heard of bosses who were able to turn on employee computer cameras and spy on them at their desks. Before you adjust those pantyhose or unzip to tuck in a shirt, best make sure nobody is watching. If the camera turns on unexpectedly, either turn it off or put a post-it over it.
Discipline: If you're written up for a conversation you thought was private, your employer may have an audio recording device in the workplace. On the other hand, the co-worker whom you thought you could trust might just have blabbed. Be careful what you say at work, and to whom you say it. Sometimes even people whom you thought were friends will rat you out.
Counter-spying: There are lots of articles written about how to counter-spy on your employer to figure out if they've snooped your computer. I can't vouch for any of these techniques, but here are some. Ctrl-Alt-Delete on a PC brings up a menu called Task Manager. If you go to the Processes tab, it will show you everything that is running in the background. When I did this, it brought up a list of a zillion things that looked like gibberish. I'd hesitate to delete anything because I don't know what I'm doing. You might want to get a computer guru to check yours if you're in doubt. If you look at your Start menu and see anything called VNC, RealVNC, TightVNC, UltraVNC, LogMeIn, Shadow, Web Sleuth, Silent Watch, or GoToMyPC, then you might have a spy. On the other hand, my computer guy sometimes uses LogMeIn to remotely repair my computer. I make sure it's turned off unless I expect him to be working on it. There are also some tracking beacons you can supposedly install in your emails to see if they're opened, such as emailprivacytester.com or ReadNotify. I wouldn't recommend any of this unless you feel comfortable that you won't crash your computer. Imagine explaining what you were doing to the IT guy.
Stuff you said privately gets repeated: Even if something you said doesn't end up in a written warning, if your boss makes a snarky comment like, "Some people think I'm tough on them. You haven't seen tough yet," right after you told a co-worker that you thought the boss was tough on you, then there's either a technological or human spy. You might try a test, such as saying something completely false but innocuous to a person whom you suspect of being a spy. Maybe share that you've decided to take up quilting or karaoke and see if it comes back to you.
Weird stuff starts when the new guy starts: I knew someone years ago who had been a hired company spy. She would be hired as a regular rank-and-file employee and get to know her co-workers to root out an evildoer. If a company suspects wrongdoing such as theft or corporate espionage, they sometimes hire a private detective or a company that will send in undercover snoops to spy on employees. If a new person is hired and private conversations are suddenly known to management, or your personal papers start to disappear or get moved around, you may have a hired spy in your midst.
Your boss comments about your personal life: Companies like to monitor social media. If you find your boss commenting about things you posted on Facebook or Twitter, you can assume that you're being watched outside the office as well.
A co-worker was snooped on: If a co-worker is disciplined for an inappropriate email, a social media post, or something they said about the workplace, you can be darned sure the company is also watching you.
All in all, the best advice I can give is to assume that everything you say, do or write at work is being monitored. Many courts say that you have little or no expectation of privacy at work. That means you shouldn't check personal emails on company devices, shouldn't open personal emails that might be inappropriate, and shouldn't do or say anything at work that you don't want to appear on the front page of the company newsletter. If you need to check personal email at work, then bring your own device to do it, and don't hook up to the company's wi-fi or server. If you're on social media, don't post anything that could get you in trouble at work.
If you assume you're being watched, you probably can't go wrong.
If you need legal advice, it's best to talk to an employment lawyer in your state, but if you have general legal issues you want me to discuss publicly here, whether about discrimination, working conditions, employment contracts, medical leave, or other employment law issues, you can ask me at AOL Jobs. While I can't answer every question here, your question might be featured in one of my columns, or an upcoming live video chat.