If you only have a volt meter, put a low value resistor in series in the wire where you broke it and measure the voltage drop. e.g. 1 ohm will give a 1 volt drop with 1 amp. i.e. if you measure 0.2V across it, the current will be 0.2A. V=IR.

Depending on the current you may need a 1W resistor or more.

The small voltage drop shouldn't affect the charger.