Below is a list of test points which can be found on Raspberry Pi 2, 3 and some are also on b+.
With the use of a multimeter, these test points can help with troubleshooting hardware issues.
I have yet to find any formal documentation about these test points. However, I do know they exist.
PP7 5V after polyfuse
PP10 Goes from 3V3 to 2V on brownout
PP11 DAC_2V5 (for composite video DAC)
PP12 AUD_2V5 (for PWM audio drivers)
PP13 Goes from 3V3 to 2V on ACT activity
PP21 RUN signal (reset)
PP22 Goes from 3V3 to 2V on activity of green (link) ethernet jack LED
PP23 Goes from 3V3 to 2V on activity of yellow (speed) ethernet jack LED
PP27 VBUS (USB 5V power)
PP28 ETH_CLK (25.000 MHz)
PP35 GPIO6 of LAN9514
PP36 GPIO7 of LAN9514
Below is an example of how you can use PP9 to confirm that the regulator is supplying 1.8v correctly.
Ground can be sourced from the SD card slot;
And the exact location of PP9;
Adafruit has some great information covering the Raspberry Pi power circuitry. link