Not all motherboards are the same. To find the correct header pins you have to refer to the motherboard manual although sometimes the motherboard labels the pins clearly enough so you can figure it out. However, with the motherboard installed in the case, it could be difficult.
If you can't find your manual, try going to the motherboard's manufacturer site to download a copy of the manual (usually a pdf format). If you have a major manufacturer made PC (Dell, HP, Gateway, Emachines, etc.), you might have to try their websites. Good info from those sites might be hit or miss.