I cannot, for the life of me, get enough power into my pi so the mouse & keyboard are operable. I have a RPi approved usb hub (Belkin: Hi-Speed USB 2.0 7-Port Hub) powering the mouse and the keyboard and the power sources I've been trying to use all claim to be 5V & 2.5A. I've tried an iPad PSU with a high quality micro usb cable, but that didn't work. I unsuccessfully tried this Canakit PSU. I bought a multimeter to measure the voltage and amps of these PSUs to see what's going on, but I could not find any useful resources that show how to measure voltage and amps on a pi 3 model B. Any help would be greatly appreciated, thank you.
You only need to check the voltage. The voltage will drop if not enough current is available.
Measure the voltage between pin 2 (5V) and pin 6 (ground) of the expansion header. It is safest to put male-female jumper wires on those two pins and touch the meter contacts to the female end of the jumper wires rather than the pins themselves.
If all is well you should read 5V +/- 0.25V.
Several vendors market the "USB Doctor", a gadget that plugs into a USB port and measures voltage and the current through its USB output. They are small and cheap but the one I bought from Adafruit seems to be inaccurate when checked against my DVM. If you are running a RPi 3 with lots of accessories, you should invest in a powered USB hub to take the load off the Pi's power supply. Also, make sure you have a heavy gauge connecting cable from the power adapter to the Pi and do not get one with a switch in-line...the switch contacts have a big drop as well.