One option is to install a security plugin. Most scan for Tim Thumb throughout your whole file structure, including themes (and will also search for many other types of vulnerabilities if you’ve been hacked, as your “hacked” tag indicates).
You can also search for files named “timthumb” in your theme directory, or search for the phrase “timthumb” in all files on your site. It helps if you have a local copy and a good editor that will search through files.