How do you check a car V?

By V, I take it you mean Voltage. Symbol for Electromotive force.Or Potential difference in Electromotive Force. Find battery. Using a Volt/Ohm Meter,that you can get from anywhere basically,walmart,lowes,autozone,to name a few.Just make sure it says DC on scale,as that is for Direct Current,what cars operate on.Black is for negative,Red is for positive.(which by the way,electron flow is from the negative to the positive). Put red meter lead on positive side of battery,and black lead on negative side of battery. Volt meter should read 12 in a perfect enviroment.10.8 or less=dead battery with car off. Car running should be 13.8 or so. If voltage is to much more than that alternator is providing to much voltage,and if you get anything under 12.6 or so you are not getting enough voltage from alternator.Or it"s a dead cell in your battery. Best thing to do is go to autozone. They test car voltage,battery,and alternator condition in less than 5 min.Oh yeah , And it"s FREE !!!