If you are on a localhost platform, you can put this in php.ini to see all the errors immediately on the screen, which is what I prefer:
error_reporting = -1
display_errors = On
The only down side is that errors in plugins and CMPs will make things hang.
If you're on a shared server, this probably won't work. Sometimes you can add this using a snippet in your template (which makes it easy to turn on and off):
ini_set('display_errors',1);
ini_set('display_startup_errors',1);
error_reporting(-1);
Errors reported by MODX itself (e.g., from sanity checks in the MODX or extra code) should go the the error log no matter what.
If your error log never shows anything, its directory may not be writable.
