|The negative terminal of the charging port may or may not be the preferred ground to use when measuring voltage. The circuit board may show the ground symbol or the etchings may indicate a large area of common voltage that might be the ground. If you have access to the board schematic it should show the ground.|
Of course once you've determined the locations you need to know what the correct voltages there should be and, if they're not right, which component is defective. That will be the hard part.