How can I use JavaScript on the client side to detect if the page was encrypted?

Question

Use window.location.protocol to check if it is https:

function isSecure()
{
   return window.location.protocol == 'https:';
}

Alternatively you can omit specifying "window" if you don't have a locally scoped location.

function isSecure()
{
   return location.protocol == 'https:';
}

As google analytics taught me:

if ("https:" == document.location.protocol) {
    /* secure */
} else {
    /* unsecure */
}

Second method for newest browsers:

var secure = window.isSecureContext;

or just get isSecureContext:

if (isSecureContext) {
   ...
}

More here: https://developer.mozilla.org/en-US/docs/Web/Security/Secure_Contexts#Feature_detection#Feature_detection