Pinout connectors

Hi just a simple question, re Hard/power led, power switch etc connectors to motherboard. I would like to know how you tell the + or - for these connectors. All motherboards are different and although they might have power, hard drive etc written on the motherboard it also has + & -. On the connectors themselves, there is no + or -, most of them have a pyramid shape, does this relate to the +,- and which one? One electrician told me to use a multimeter on these connectors to see which one is which but that seems like a lot of trouble. Can anyone tell me how to identify the + and -, thank you