Simple download Web Shell Detector from github. And then upload shelldetect.php and shelldetect.db to your root directory. Open shelldetect.php file in your browser or execute it under command line.
In case file was indicated as "suspicious" you may submit it to shelldetector.com team to manual examination, after carefull analysis, you will get report.
With wide range of settings. You able to create needed behavior.
PHP 5.x, Python 2.x, OpenSSL (only for secure file submission)