First you need a digital volt meter , set to a DC voltage range of 0-20v a hobbyist meter like this will be fine.
then you need to connect the leads of the meter to these 2 pins on the pi
Now you can try and do it connecting the lead directly to the pins but I don't advise it as you can easily short out adjacent pins
so I recommend you use 2 dupont wires to connect to the pi gpio pins and then connect your meter to the dupont wires.
now ideally you want a voltage of 5 - 5.1V showing on the meter with the pi booting. in this example I am boot my pi3B using a 5V 2A switch mode power supply and as you can see I have a voltage of 5.055V which is just about perfect.