Back face, front face?

[L i am]

This is such a newby question but I am playing about with some tricky lighting. I need to apply something to the back face of a material. I rarely if ever consider which is the front face or back face. So how do I tell which is the front and back face in a model? I am aware that when I create a virgin face the front is white whilst the rear is grey
Thanks

[Box]

In a well constructed model all front faces would be facing out as you would be managing them as you go. If you are in the habit of painting everything early on you may well be covering any incorrect orientation.
So simply put, change the style to monochrome and see what is white and correct what isn't.

[L i am]

Thanks is there any vertue in applying materials to the front face instead of the backface? Just trying to further my understanding

[Box]

Many render engines will ignore back faces, this causes holes in your renders.
3d printers use the face orientation to know what is air and what is print media.
I cannot think of a reason why having back faces painted and facing out, would be beneficial.
Some times it can be useful to paint back faces because you want the inner space to be a colour or hide 'gaps' that can appear when edges are turned off, and transparent materials can be useful on back faces on occasion.