The best indicator is the green / blue light (different depending on the version) LED on the HDMI board (the board closest to the UV light, see photo below). Here is a description and possible causes ...
HDMI
LCD does not show anything and there is a blinking light on the HDMI board
When the light on the HDMI board is blinking that means the board is not receiving any signal from the computer (or RaspberryPi). Check that the HDMI cable is connected securely Check that the PC is s...