Simple battery testers that will plug right in to your charging port and measure battery voltage both loaded and unloaded are available from most hobby suppliers. Unloaded means the meter is checking the battery's voltage with no load on it. Loaded means a switch on the meter enables a circuuit that draws roughly the same amperage as a couple servos. If the loaded reading drops below 4.8 volts, (6 volts depending on your setup) don't fly it. The voltage will be seen in a digital read out and these meters cost about $20.00. Or simply put a volt watch on the side of the plane. A volt watch is a small electronic device with a series of LED lights on it. There will be about seven or so lights. When the battery has enough current in it to fly a plane, the green lights at one end of the volt watch will be on. As the battary loses voltage, the yellow lights in the center of the volt watch will come on. As the battery comes closer to not being able to fly a plane, red lights at the opposite end of the green lights will come on. If you have red lights on, don't fly the plane. Volt watches do no show actual voltage in numbers, but it is a rough indication of whether or not to fly. These things are plugged into an open port on the receiver and are active any time the plane's on/off switch is in th "on" position. I noticed recently that these things had increased to about $12.00 to $15.00 each, but they can be found on the internet for much less.