I have a strong suspicion that the screen of my laptop Lenovo Thinkpad 450s is broken. If I plug an external monitor, I can use my laptop, and everything seems to be fine.
How can I confirm that the screen is dead? If I go to Settings->Displays on Ubuntu 18.04, the internal laptop screen does not show up as an option. But is there a more formal way to confirm that it is an hardware problem, resp. to diagnose where the problem is?