Enough talk, let's get started.
Here are some example sites for phpfox I've found by now:
How did I found these sites?
Google: "powered by phpfox". This is the standard keyword to begin a search for all platforms. If you don't find anything related to your platform you should search for a demo site for that platform and try to figure out some unique footprints on that sites.
In this case I've noticed that phpFox has unique footprints for all releases they publish.
Now we have a good chunk of footprints to begin with:
"powered by phpfox version"
"Powered By phpFoX Version 1.5.1."
"powered by phpfox version 1.6"
"Powered by phpFoX Version 1.6.20."
"Powered by phpFoX Version 1.6.21."
"Powered by phpfox version 2.0"
"Powered By Phpfox Version 2.0.4"
"Powered by phpfox version 2.0.5"
"Powered By phpFox Version 2.0.6."
"Powered By phpFox Version 2.0.7."
"powered By Phpfox Version 2.1.0"
"Powered By phpFox Version 2.1.0beta2"
"Powered By Phpfox Version 2.1.1"
"Powered By phpFox Version 3.0.0."
"Powered By phpFox Version 3.0.0beta1."
"Powered By phpFox Version 3.0.0beta5."
"Powered By phpFox Version 3.0.0rc1."
"Powered By phpFox Version 3.0.0rc3."
"Powered By phpFox Version 3.0.1."
"Powered By phpFox Version 3.0.2"
"Powered By phpFox Version 3.2.0."
"Powered By phpFox Version 3.2.0beta1."
"Powered By phpFox Version 3.2.0rc1."
"Powered By phpFox Version 3.3.0"
"Powered By phpFox Version 3.3.0beta1."
"Powered By phpFox Version 3.3.0beta2."
"Powered By phpFox Version 3.3.0rc1."
Next thing I like to do is to take a look into the URLs and registration form of these websites. Footprints like "Sign up for phpFox" or "/index.php?do=/user/browse/" (= inurl:"/index.php?do=/user/browse/") would be just fine, if you get good results in google. As both footprints gave me not as much results as I hoped for I leave them out for now.
Once we have figured out some footprints we use the most common one "powered by phpfox" for detection as GSA needs to know what kind of platform it deals with. As some sites will hide these footprint or the footprint isn't as unique as it should be it is always a good idea to investigate the source code of the platform. The best results we would get to identify a platform is to find a code snippet that is unique and a component of every homepage that is driven by that platform engine.
You can open the source code with "ctrl+u" (in chrome). I have found "content="phpFox"" and "Phpfox.init" in some of those sites and these snippets will help for the beginning.
Copy and paste this script into your text editor and save it as "phpFox.ini". This little script is very useful right now as it helps to find and identify websites powered by phpFox.
Next you have to do is to paste phpFox.ini into your GSA engines folder (...GSA Search Engine Ranker\Engines). You can scrape for phpFox sites with the "Search online for URLs"-Tool (Options -> Tools -> Search online..) now. Just choose your footprints by "Add predifined footprints" -> MyEngines -> phpFox. It is helpful to save the results to a custom file as well as the unknown ("Save Unknown") once scraping is done. The unknown sites can help to improve your script later and to identify more of these platform.