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.
PP3 GND PP4 GND PP5 GND PP6 GND PP7 5V after polyfuse PP8 3V3 PP9 1V8 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 PP14 SD_CLK PP15 SD_CMD PP16 SD_DAT0 PP17 SD_DAT1 PP18 SD_DAT2 PP19 SD_DAT13 PP20 H5V 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 PP24 COMPVID PP25 AUDIO_L PP26 AUDI_R PP27 VBUS (USB 5V power) PP28 ETH_CLK (25.000 MHz) PP29 VC_TMS PP30 VC_TRST_N PP31 VC_CLK PP32 VC_TDI PP33 VC_TDO PP34 GND PP35 GPIO6 of LAN9514 PP36 GPIO7 of LAN9514 PP37 CAM_GPIO0 PP38 CAM_GPIO1 PP39 SCL0 PP40 SDA0
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