How do I tell if employees are using TeamViewer at work?

The TeamViewer website states that TCP port 5938 might be used, and that port seems to be open on my local machine when TeamViewer is running.

Trying running a port scan looking for machines that are open on port 5938.

I've also noticed that the built-in web server in TeamViewer replies "This site is running TeamViewer.", so do a port scan on port 80 as well and check the results. Nmap is a nice tool for port scanning.

Try command:

avahi-browse -t -r _teamviewer._tcp

You'll get a list of IP addresses:

eth0 IPv4 850808873        _teamviewer._tcp     local
hostname = [xxxxxx.local]
address = [172.16.0.81]
port = [2020]
txt = ["UUID=bd5064d1-5ec8-496c-93cd-c273ec37faa7" "Token=nHLKXaM19dWptBDw" "DyngateID=850808873"]