How to find the positive wire on wall charger
If you don't have a multimeter or LED you may consider an alternative method:


Take a glas of water and add a teaspoon of salt 
Insert the two wires into charger's 9V output plug 
Dip the other sides in the glas and keep them at least 1cm apart 
Bubbles will appear


=> The wire where you observe the densest bubble stream is the minus pole. 

You way also want to watch this or this.