this may not be the correct way or maybe it is ......
i just put my red + from my multi meter on the positive post of my battery and unsurprisingly the black negative - to my negative post of my battery and if i recall correctly was getting about 14 volts which is right on target i believe.....
you can buy a mutitester at harbor freight for under 6 beans... sometimes they are like 3 bucks on supe special sidewalk sales and the like....
https://www.harborfreight.com/7-Func...ter-63759.html