There's no foolproof method; some student somewhere could have scanned it and put it on her college web page without announcing it anywhere. However, there are some regular places to check.
It may sound obvious, but you should always look in the PG archives first. Download GUTINDEX.ALL and keep it handy. Search the InProg List [B.1].
The main place to search for your book is the On-Line Books Page <http://onlinebooks.library.upenn.edu/>, which specializes in indexing books that people make available on-line.
If you still don't see your book on-line anywhere, hit your favorite search engine, and give it the title, author's last name, and preferably a few uncommon words from the first page of the book. Sometimes one of those solo efforts shows up in a general search.