A helpful tool for detecting Mixed Content on your website is the Mixed Content Scan, a PHP CLI script created by me.
To run this script from the command line interface, simply use the following command:
$ mixed-content-scan https://www.bram.us/
While the script runs, it will continuously scan and provide feedback. If any Mixed Content issues are found, the URLs causing these warnings will be displayed on your screen:
$ mixed-content-scan https://www.bram.us/
[2015-01-07 12:54:20] MCS.NOTICE: Scanning https://www.bram.us/ [] []
[2015-01-07 12:54:21] MCS.INFO: 00000 - https://www.bram.us/ [] []
[2015-01-07 12:54:22] MCS.INFO: 00001 - https://www.bram.us/projects/ [] []
[2015-01-07 12:54:22] MCS.INFO: 00002 - https://www.bram.us/projects/mint-custom-title/ [] []
[2015-01-07 12:54:23] MCS.INFO: 00003 - https://www.bram.us/projects/bramusicq/ [] []
[2015-01-07 12:54:24] MCS.INFO: 00004 - https://www.bram.us/projects/gm_bramus/ [] []
[2015-01-07 12:54:24] MCS.INFO: 00005 - https://www.bram.us/projects/js_bramus/ [] []
...
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1085/1217158084_a9b059d25b.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1040/1216293529_3b7c044815.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1029/1084232736_5b8c023f46.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1318/1043062251_17071a8cc7.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1221/1043059543_05713e6156.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://www.google-analytics.com/urchin.js [] []
[2015-01-07 12:54:47] MCS.INFO: 00041 - https://www.bram.us/2011/09/30/css-regions-and-css-exclusions/ [] []
[2015-01-07 12:54:47] MCS.INFO: 00042 - https://www.bram.us/2014/06/04/good-looking-shapes-gallery/ [] []
...
In addition to scanning individual URLs, you can also input a file containing a list of URLs for scanning purposes. Furthermore, you have the option to generate JSON output and utilize ignore patterns if needed.