volt watch meters
They usually come in 4.8 volt and 6 volt models.
You should be charging your Nitro Battery Pack(s) the night before a race, so unless your racing in an Enduro or Bashing hours at a time, it probably isn't necessary.
A 1000 mah Pack should last at least 1 hour total on-time, even with Digital Servos. (this assuming the servo endpoints are set correctly)
If you still feel the need to use one, they are relatively cheap at about $15 usd.