MacBook Pro 'retina' 13-Inch
To open the Mac you need a P5/PL4 Pentalobe screwdriver.
The motherboard number is on the bottom left corner, hidden under two cables by Apple.
Plug this in every time you work on this Mac.
it will show you if the CPU/PCH is running (Green LED) even without booting into an OS, The two Red LEDs show if the Mac is on and if the 5volt line is correct. LINK
This little board works as a 'Known Good' battery. It makes it easy to see if the SMC and the ISL charging chip are working.
It also allows you to test the Mac using a power supply through the battery port. This way you can test the Mac even when the charging circuit is bad.
The easy way to see if the Mac is producing video. When the internal LCD panel is not connected or the lid of the laptop is closed this Mac will output video to the HDMI port. LINK