How can I tell how much juice a LiPo battery still has in it?
To be honest, if you manage to find a way to do this reliably, just measuring the voltage, and patent it, then you will never have to work again.  The available energy left in a battery is loosely related to the terminal voltage but also depends on the battery temperature.

A common method of determining the battery charge state is to use a coulomb counter to count the charge going into and out of the cell.  This gives a better estimate of the charge state of the battery, although the actual energy available is still temperature dependant.  At low temperatures the battery capacity can be less than 50% of the nominal.  An example device is the ST STC3100.  This uses an I2C interface to communicate with the processor.  The coulomb counting is performed by integrating the current in and out of the cell over the charge/discharge cycle.  If you know when the battery is full then you can estimate the amount of charge that has been used.