You can not reliabley tell the state of charge by reading the voltage. The only way to know for sure is to do a controlled discharge cycle where you control the current and measure the time it takes to drop the voltage to the cuttoff voltage which is typically 0.9 volts/cell if you want to compare against factory specs. Some chargers like the Triton, Hobbico Elite and several others allow you to set this up and do it automatically. If you are charging at 0.1C for 12 to 16 hours, you can be sure that the batteries have obtained all the charge that they are capable of. Check out www.rcbatteryclinic.com for some good info on the care and use of all types of batteries.