if the car is running should show about 14v as TerribleONE mentioned above.
i suppose if you kinda want to read your battery volts, just turn the car in the ON position with the a/c and stereo turned off. it's still draining power though with the ECU and pump and stuff running..
but it's really the cca dat will tell you if ur battery is good or not.. you need an ammeter for dat.