Quote - > Quote - > Quote - How determine exactly wich part of the body emit more or less specular ?

You shine a light and measure the reflected light (from several directions for both).
If you don't have the equipment to collect the data directly, you can watch films, or view photgraphs to get a rough idea (certain areas of the face tend to be naturally greasy/oily).

For that we need a specular mask, not a color map converted in greyscale as usually used in some textures set i think ...

Correct, there's no way to cheat as is done for a lot of bump maps, so a specluar map has to be properly engineered.