Is there a method to search the web for website page sources that mentions Wordpress, vbulletin, or Yahoo Site builder? Basically websites using wordpress or something like Yahoo Site builder?
you scan the page for something that is common throughout all installations of your target CMS or script, for wordpress there would most likely be something in the <head> I know that wordpress adds a XMLRPC meta tag you could look for, as well as some themes have "proudly presented by wordpress" in the footer, which could be another footprint.
If you want to know if a website uses wordpress check out this website: http://www.whatwpthemeisthat.com it will tell you the plugins it is using, theme, version etc. There is also a chrome extension called Wappalyzer that will give you a more in-depth analysis on what exactly the website is using.