Detect PHP script usage in themes?

You can look at the source code and search for common URLs to such a script. But that will just show you something useful when a theme is using that script. If you find nothing, the script might still be part of the theme code.

Themes on wordpress.org are not allowed to include TimThumb (as far as I know), so they are safe for your specific use case, at least the themes reviewed and/or updated during the last two years.

There no list of themes (not) including a specific script.

You should search for a theme you like, then ask the author or, if the source code is available, search in the code.