ok, i’m not having a go here I just want to point something out:
I would understand where you are coming from if people were in fact being deceptive on a regular basis
(or at least doing what you say without mentioning it) but…
In my humble opinion; I think most artists DO explain if they use any 3d or photographs in the artwork. If they put down only photoshop, then I am inclined to believe that they used only photoshop (and no photo paste jobs).
Linda B (Enayla) was accused of using photographs in her pieces I recall. The accuser just could not accept that someone could paint what she had painted, well… thats HIS problem, not Linda’s ;). Linda and others have been kind enough to post many tutorials on how they do their work, these are the ‘tricks’ you are requesting. There are no super tricks that they hide from everyone.
Again, a skilled painter does not need to use pasted photo’s for textures, they can paint it.
They do not need to render the scene out in 3d to get the perspective, they can paint it.
Not that I am putting down people who do use other tools. But I just don’t see this ‘hiding of secret tools’ as something that is wide spread. :shrug: 
As with anything, there are a minority that will try and pass off photo’s as hand painted work but they usually stand out and have inconsistant work (i.e the non photographic elements look crap or the blending is poor).
Just because something looks very real or complex does not mean the painter had to use 3d or photographs. So I maintain that 99 percent of those pictures you think must have had more than photoshop are likely to be just photoshop
There is no secret 2d artist handshake to gain access to a wealth of quick fix tools and magic bullet techniques 
Walrus had it spot on, just ask the artist. 
Good luck,