Apple - How to know if Mac OS X machine is being remotely viewed?

If your computer is being remotely accessed, it will show a little viewer icon in the menu bar. (Note, I've been using screen sharing since OS X Leopard, and I've never seen the icon noted by de_an777 in his answer.

Go into System Preferences > Shared. Make sure that Screen Sharing and Remote Management (for Apple's Remote Desktop) are both unchecked.

Also, check under Security & Privacy > Firewall and turn the Firewall on. Note the warning. "The firewall will block all sharing services, such as file sharing, screen sharing, iChat Bonjour, and iTunes music sharing. If you want to allow sharing services, click Advanced and deselect the “Block all incoming connections” checkbox."

This will block any incoming screen sharing connection (as well as other services).

To check to make sure that you can't connect to your computer via screen sharing, you can use nmap, a free command line tool for "network discovery and security auditing."

To use it, just type nmap [YOUR IP ADDRESS]

You'll see that nmap reports that the vnc (screen sharing) port is open. After turning off screen sharing and turning on the firewall:

(Note that I've explicitly allowed ssh, printer, and afp sharing in the Firewall.)

I hope this helps you!

Interesting, I just had the same annoyance. IT logged in, moved my mouse, viewed a few things and I thought, "hang on, there's no indicator at all this is happening..."

The other answers seem correct, however I had an additional thought - this checkbox.

3rd one down "Show when being observed". This was unchecked, presumably from when my machine was originally set up.

My machine is <1 month old, a replacement/upgrade, I have admin rights but still need IT for a range of things. I was/am spewing that this was the default setting.

I came here because I noticed my mouse pointer moving around in a distinctly human way without me moving my mouse. I realize this isn't strictly an answer to your question but it may help someone:

It turned out that my office Mac had previously been owned by other people in my office, and was still paired with their Bluetooth mice. Their mice sometimes connected to my machine and then they were waving their mouse around trying to figure out why it wasn't controlling their own computers.

You can unpair mice by going to the mouse settings and clicking the "set up Bluetooth mouse" button, then clicking the "X" next to devices you don't recognize as your own.