You can't use a lipo for your receiver unless you also install a regulator. I don't know about your transmitter's voltage requirements so maybe you can use one there or maybe not. If your charger can charge LiFe batteries you can use them as a receiver pack with no regulator. I'd actually say that's the best option all around, but it does require buying an LiFe compatible computerized charger which many new pilots don't want to do. The standard thing is to use NiCd's for both Rx and Tx. They are durable and don't need a fancy charger, although you'll do well to cycle test them before flying with them just to make sure you didn't get a dud.

Buying the $9 meter is the way to go if you don't already have an electrician type multimeter. Any battery needs to have a load on it to give you a meaningful test. So a loaded voltmeter has a resistor in the testing circuit so that you can put a load on your battery (usually 1/2 amp) to see if it's delivering the voltage it should under load. If you don't have a loaded meter, you can use a regular non-loaded meter with a soldered up extension by plugging it into an unused receiver channel and flexing your elevator or rudder. Putting that weight on the servo creates an electrical load, which lets you get a reading of what voltage your receiver is actually seeing.