Let's see. This is tough. But, I would recommend before trying anything else to run Apple Diagnostics if you already haven't.
https://support.apple.com/en-us/102550
And try reinstalling macOS: if you already haven't.
Other than that, I don't know many other ways to test the display. You could also try going to Settings ( > System Preferences > Displays) and while holding the Option (alt) key, choose "Detect Displays". Honestly this has never worked for me, but it's worth a shot.