if the wire is burnt than the melted wired would increase resistance throughout the whole wire. set your dmm to ohm and grab a strand of wire and put your meter on it it will be around ~0. than add a wire to a ground and than meter it. there will now be resistance like if you grounded out a live wire. if you check any mecp book it'll tell you that you should test with a dmm. depending on how accurate your dmm it'll give you a great start.

also testing power at the hu and than at the amp will help tell you that if the wire is bad or if you messed up your remote wire on your hu.

what is your reasoning that you can't test for a bad ground/wire with a dmm?