HTTPS Checker scans the HTTPS domains you specify to detect mixed content issues (HTTP files referenced in an HTTPS page).

Usage is straightforward. Type a domain, click "Run" and watch as the program detects and displays issues. (Note, the free version scans up to 100 pages only.)

When the process is over, HTTPS Checker organises any issues it's found into various categories.

SSL Certificate validation
Active Mixed/ Insecure Content
Passive Mixed/ Insecure Content
Insecure Forms
Insecure Redirects
Insecure Canonical Links
Insecure Links to the same domain
Insecure Social Links
Insecure Sitemaps
Warnings. Examples include "You are not currently using the "strict-transport-security" header, which can leave you open to man-in-the-middle attacks with tools like SSLStrip" and "Though this site has HTTPS, the HTTP version does not actively redirect to the HTTPS version".

You're able to expand particular sections of the report to discover the affected URL(s).

HTTPS Checker is also available in various commercial forms. Paying from $9-$99 a month gets you support for scanning 10,000-250,000 pages in one session, detailed downloadable reports, a user agent switcher, more crawling options, an online tool, and more.

Verdict

HTTPS Checker does a thorough job of checking for mixed content issues, but the free edition can only tell you about them in a very basic report.