TimThumb is not a WordPress plugin.
It is more commonly a part of themes and other WordPress plugins, so you won't know that your TimThumb is out of date. You have to trust that the WordPress plugin creators provide an updated version.
Unfortunately, many of the plugins and themes using TimThumb are commercially paid editions which are not managed directly by WordPress' own plugin database, you download and install them semi-manually or fully manually.
Also, these plugins and themes rarely publish which TimThumb version they use, they don't publish security advisories or notes regarding their products, and and and.
Nevermind that the entire concept of TimThumb is b0rken, technically speaking. :)
Generally, allowing pluggable PHP code is a Bad Thing security wise.