The starting point to diagnosing a working Series X console is the top daughterboard. Here is a just a dump of my notes working on it and important information until I form more formal documentation.
The top board where the small 2 pin 12V power connects, containing the Southbridge (Microsoft M1139994-001 T6WD5XBG-0004 Southbridge I/O controller), ethernet, audio and some regulators (3.3V, 5V, 2.5V and 1.8V on some revisions).
Start your diagnostics with this board. Connect the 12V power supply or a 12V from a bench power supply to the connector pins.
Measure near the connector for the regulator outputs (on the large inductors and capacitors. You should have 12V on the capacitors closest to the 12V input, then around the board in various places (I will do photos of the exact spots later) you should find the regulators outputting 3.3V, 5V and 2.5V, and on some early revisions a small regulator outputting 1.8V.
If you have all the above regulators working, next connect the power button ribbon to the board and press the power button, or alternatively short the 4th pin from the left to ground if the connector is on the bottom left of the board as you are looking at it. This is a pin that is pulled to 3.3V and shorted to ground when the power button is pressed.
If the Southbridge is working and the regulators are working, you should get the white line for about1 second then it turn off again. If you do not get white light there is something wrong with the daughterboard.
Do not bother connecting the daughterboard back to the motherboard until you get the 1 second white light boot.
NOTE: Daughterboards are independent of the motherboards and so long as the main ribbon connectors are the same (they change between revisions), you can use a daughterboard from one Xbox on another Xbox no problem.