This will check all PHP files in your site that have been modified in the last 30 days. Just be sure to replace /path/to/your-site with the actual path to your site as you probably can imagine. You can also change php to a different file extension to search more thoroughly.
find /path/to/your-site -type f -name "*.php" -ctime -30
Source: http://premium.wpmudev.org/blog/removing-backdoor-exploits/