Most common browsers include an indication in the bottom of the window when you
are communicating with a secure server: 
When a secure server is indicated, data that you send and receive is encrypted,
which prevents anyone but you and the server from seeing the information
exchanged. The protocol by which this takes place is called SSL - secure
sockets layer.
In order to ensure that your SSL connection is to the actual server you intend,
certificates and certificate authorities are used. The secure
server and your browser automatically exchange certificate information when
establishing a connection. If the certificate can be authenticated by a trusted
certificate authority, the secure page will come up automatically.