Get a LOADED voltmeter. When you plug it into the recharging plug/port it will give you a reading. I am the cautious type, so I will not fly if the reading is under 4.6 volts (fully charged it should read about 5.2 - 5.4 volts ). There are others that will fly with a lower voltage.

If you charged your batteries the night before and took them off the charger the morning you are going flying, I would check the receiver battery after the 2nd flight. If the receiver battery is over a year old, I would check it after every flight to be sure it is OK.