Steve Gibson is well known for his free tools for checking Internet security. His site at https://www.grc.com/has long been a place to go for a variety of security information. In the light of all the recent revelations about government and other institutions spying on us, his page for checking if someone is eavesdropping on an HTTPS connection is worth visiting. Go to the link called Fingerprints and learn if your ISP or other agency is using a fake SSL certificate to look at your supposedly secure HTTPS encrypted traffic.
Gibson gives many details about what can happen with secure browser connections and how to check the fingerprint of these connections. He provides an online tool to obtain the genuine hexadecimal fingerprint of an HTTPS domain. He also gives instructions for a number of browsers on how to find what SSL certificate fingerprint is being given to your browser. I have used the online tool to obtain the certificate for Gizmo’s and you can see the result by clicking the thumbnail.
Gibson's page is informative and worth visiting.