Do you mean for projection painting? If so, I don't think you can, unfortunately (I wanted my normal map shown as a painting guide once). If you need for for this, I think I applied my normal map on a different layer in the colour channel, and used that as a rough guide. I don't believe it shows alpha maps either.
I have always been under the impression that you Can't see your normals real time in bodypaint. You have to render to see them.
So when I'm working on my normals, I just basically have it in the diffuse slot and edit the normals that way. It can help a lot seeing the normal in 3d when you can't pick out a specific error in 2d.
If you could see the normals real time and edit them though, that would be really cool. As far as I know, the only program capable of this is Mudbox2009.
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Otherwise, just turn on Enhanced OpenGL.
So when I'm working on my normals, I just basically have it in the diffuse slot and edit the normals that way. It can help a lot seeing the normal in 3d when you can't pick out a specific error in 2d.
If you could see the normals real time and edit them though, that would be really cool. As far as I know, the only program capable of this is Mudbox2009.