On Ebay, there are literally hundreds of lipo monitors. Some have displays and will tell you the voltage of each cell within the battery, some are just low voltage warnings with a buzzer. Most are from china and are really quite cheap - well worth the meagre expense certainly.

The more advanced TX have displays on them, most of which will tell you the voltage of the TX battery.

You could also check the voltage of your pack prior to running. A typical TX lipo (any 3s that fits) would give quite some runtime so you could just keep it at a reasonable state of charge by charging it up to storage levels after you run. Lithium batteries prefer partial charges (as opposed to running them till cutoff then fully recharging) anyway. Disconnect the battery from the TX whilst it is stored.