It depends on your operating system and/or web server. I am able to use ssh on my web server and it is running Linux so I go to the themes directory under wp-content and do a command line search as follows:
find . -type f -exec grep WP_Widget \{\} \; -print;
This will only work in Linux/Unix. If you use Windows and have access to the web server you can use the windows file content search to find “WP_Widget” in the above mentioned directory.