Ah, good question. Usually I’d go by what the fans are doing, but seeing as you haven’t got any… 
Okay, there are a couple different “any time” blink codes and a couple that happen at specific times which you might be able to use as a diagnostic.
Any time:
- red-blue-off: battery disconnect is active
- red-blue-off: battery is not detected, standalone mode is OFF
- red-off-off: chassis is open (C cover (midplate) removed)
- red-amber-off: GPU bay open
- red-amber-off: GPU module fault
- red-blue-off: input deck not fully populated
These are assessed in order. The first one to hit wins.
Outside of that, you’ll get “diagnostic” codes. These only happen when the power is on and the AP firmware has started. They override the blinking codes from above.
15 seconds after you press Power and only if anything fails to initialize, you will get a series of 1-12-1-8-1 blinks in different colors.
If you don’t get those blinks with the chassis open and all of those components out, you are not booting far enough for anything to matter. 