You can use Mixed Content Scan, a PHP CLI scripts I wrote which scans your site for Mixed Content.
Run this script from the CLI, a such:
$ mixed-content-scan https://www.bram.us/
The script itself will start scanning and give feedback whilst running. When Mixed Content is found, the URLs causing Mixed Content warnings will be shown on 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 12:54:26] MCS.INFO: 00006 - https://www.bram.us/projects/js_bramus/jsprogressbarhandler/ [] []
[2015-01-07 12:54:27] MCS.INFO: 00007 - https://www.bram.us/projects/js_bramus/lazierload/ [] []
[2015-01-07 12:54:27] MCS.INFO: 00008 - https://www.bram.us/projects/the-box-office/ [] []
[2015-01-07 12:54:28] MCS.INFO: 00009 - https://www.bram.us/projects/tinymce-plugins/ [] []
[2015-01-07 12:54:29] MCS.INFO: 00010 - https://www.bram.us/projects/tinymce-plugins/tinymce-classes-and-ids-plugin-bramus_cssextras/ [] []
[2015-01-07 12:54:30] MCS.INFO: 00011 - https://www.bram.us/projects/flashlightboxinjector/ [] []
...
[2015-01-07 12:54:45] MCS.INFO: 00036 - https://www.bram.us/2007/06/04/accessible-expanding-and-collapsing-menu/ [] []
[2015-01-07 12:54:45] MCS.ERROR: 00037 - https://www.bram.us/demo/projects/jsprogressbarhandler/ [] []
[2015-01-07 12:54:45] MCS.WARNING: http://www.google-analytics.com/urchin.js [] []
[2015-01-07 12:54:46] MCS.INFO: 00038 - https://www.bram.us/2008/07/11/ror-progress-bar-helper/ [] []
[2015-01-07 12:54:46] MCS.INFO: 00039 - https://www.bram.us/2008/11/10/jsprogressbarhandler-033/ [] []
[2015-01-07 12:54:47] MCS.ERROR: 00040 - https://www.bram.us/demo/projects/lazierload/ [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1212/1285026452_0aeb38b6e6.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1074/1273115418_a77357040a.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1096/1273106588_91f7a736c6.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1324/1216309045_31ca82f9d9.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1262/1217169586_e4b2bfa7df.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1149/1216304291_63fd48d9c4.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1366/1216301505_51b3c590ff.jpg [] []
[2015-01-07 12:54:47] MCS.WARNING: http://farm2.static.flickr.com/1184/1216299847_c57975bed2.jpg [] []
[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/ [] []
...
It's also possible to pass in a file containing a list of URLs to scan, and to change the output to JSON. Ignore patterns are also supported.