It depends on the quality of the convertor and how much current the board takes through the 20/24 pin connection.

Voltage drop due to adaptor = connection resistance x current used by board

You can get a multimeter from electronic component shops or hardware shops dealing with electrical items. Digital ones are recommended (they give a readout calculator style), even cheap ones (<RM30) are on average more accurate than most motherboard sensors. Analog ones (look like car speedometers) are useless for measuring PC voltages, as they are not accurate enough.