Ensure debug mode is on - either add APP_DEBUG=true to .env file or set an environment variable
Log files are in storage/logs folder. laravel.log is the default filename. If there is a permission issue with the log folder, Laravel just halts. So if your endpoint generally works - permissions are not an issue.
In case your calls don't even reach Laravel or aren't caused by code issues - check web server's log files (check your Apache/nginx config files to see the paths).
If you use PHP-FPM, check its log files as well (you can see the path to log file in PHP-FPM pool config).