How do you know when your capacitor is charged and ready for hook up?

The capacitor charges up when you initially install it and apply 12 volts to the circuit. Its purpose is to smooth out power drops in the 12 volt supply line to the amp. For instance, when the bass hits the speakers, the amperage surge can exceed the amperage the line delivers for a split second, so the capacitor supplies amperage briefly to sustain the power.

Hooking up a small motorcycle battery close to the amp serves the same purpose but is possibly messier because of the potential to leak acid.