I found this, it's a neat article speaking about spec maps
http://www.manufato.com/?p=902
But i found this particular trick to be especially usefull 
(of course there is no influence from the normal map ;/ but still cool for a quick look)
Now, lets make this a little more interesting…What about having a preview like this one here, in Photoshop?.
[Blocked Image: http://www.manufato.com/wp-content/uploads/2010/10/dielectric-and-conductor-an.gif]
To do that is actually quite simple.
Bellow the diffuse layer, in Photoshop, create a group and make it multiply.
In this group add two new layers. The bottom one you will fill with black. On the top one you will create some white spots, faking the shape of the specular reflection that you would expect to see on the surface.
[Blocked Image: http://www.manufato.com/wp-content/uploads/2010/10/dielectric-and-conductor-l.jpg]
Now, you just need to drag the spots around to see how the specular reflection works for this material.
[Blocked Image: http://www.manufato.com/wp-content/uploads/2010/10/dielectric-and-conductor-an.gif]
Pretty cool, right?
Of course, I don’t keep these layers visible all the time while working on the specular texture. Sometimes I just don’t use them at all. But every time I want to be sure I’m heading on the right direction I’ll toggle them on and off, and move the small white spot around, to see if I’m still on track.
