There is no easy way, just a slow & methodical test of each & every section of wire and componant.
There is some expensive equipment that has evolved from the gear invented to chase earth shorts in electric fences and you can also use really sensative thermal gear to see where heat is being generated
Other than that, start with a circuit diagram.
Remember there are 2 sources of power, the battery & the alternator .
So follow the power flow from the battery to where the wires melted
Or follow the power flow from the rectifier to where the wires have melted.
If it is a short then problem will be usually be between the last thing in the flow that melted and the first thing that did not melt.