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 based on the HDMI LED:

Basically the green indicator LED on the HDMI board has a few states:

• No light = no power, either the fuse onboard is blown or the system is not sending 5V power to the USB cable that plugs into that board. Test by using a different cable powered from a USB port on the computer.
• Flashing= board has power, but incorrect HDMI signal or no signal is present. Check the HDMI cables, the adapter cable plugged into the back of the board and the PC. You can also bypass the adapter cable and plug the HDMI directly into the back of the board for testing.
• Solid= good valid HDMI signal is being sent to the board.