how to detect "other software that intercepts the connection and act like a man-in-the-middle"

Question

I checked ubuntu proxy settings, nothing set.
Checked also VPN, nothing set.

If I try to click on a video on a google link that opens youtube, using firefox, I get this message:

Secure Connection Failed
An error occurred during a connection to www.youtube.com. PR_END_OF_FILE_ERROR
    The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.

(there is no problem if I open the link in a new tab)

I got the man-in-the-middle tip from: https://support.mozilla.org/en-US/questions/1267074

I think there is no hack attack... but I got curious if there could have some way to determine this man-in-the-middle other than proxy or VPN settings. May be checking all the in between connections/hosts that happen from my machine to youtube? I forgot the command that does that, not sure if useful tho.