How to best diagnose a dead screen vs. the graphic card

I have a 2013 iMac 27" which suddenly started to display a blank / black screen. It is definitely running as I can hear the chime at power on and the fan and hard drive making noise. I have tried to boot into reset / diagnostic mode and check that the ram was seated correctly, still nothing.

I should mention that an external monitor will not work either and is the reason why I question if it is the screen itself or the graphic card...

What is are there best methods to diagnose the graphic card vs display? Can I run any diagnostics without removing the screen?

Thank you,