The EZist and best way to check the battery is to take it to Autozone or Advance Auto and have it tested/ checked
To answer your exact question: pop the caps on top, BE CAREFUL, THERES ACID IN THERE. USE SAFETY GLASSES. Each cell should be about 3/4 of an inch from coming out of the top. Use distilled water to refill any low cells
It's been my experience, that if they are real low, the battery will not RECOVER, and you will need a new one.
"Gravity" can be tested with a tool from the parts store. If you are going there for that. Take your battery and let them test it.
Also THIS DOES NOT "RECHARGE THE BATTERY". Just gets it ready for your home charger or alternator to charge it.
Do you have a Voltmeter, and are not Scared to use it?
What is your car doing---or what did it do when you 1st had the NO START ISSUE?
DID IT TURN OVER?
DID IT TURN OVER AND NOT FIRE UP?
DID IT SPIT AND SPUTTER--OR WAS IT JUST STEADY GRUNTIN'?
Please use mega detail (I did this, and my car screamed like a little girl, or I did that and the left front wheel fell off----'cept tell me the real deal!)
Might can get you through this if you are capable!
Let me know (below). Good-Lookin' Assistance is on Standby!