Problem
You want to know the application bootstrap file.
Solution
Use the App::getBootstrapFile().
This returns the full path to the bootstrap file.
echo App::getBootstrapFile();
Will output something like.
/apps/example/vendor/laravel/framework/src/Illuminate/Foundation/start.php
Discussion
This is a low-level method.
You can see where this file is loaded in Understanding the Request Lifecycle. In The Booting Steps, it's the block right after "Starts Application" labeled "laravel/start.php".
