I just thought I would share an exercise I do to help me see what needs to be retouched, as many of you know what makes you a good retoucher is seeing what needs to be done before you pickup a tablet, photoshop can be learned relatively easy, knowing when to use the tools is another thing. Practice and critique I believe is another key part but this helps me out:
Find a collection of high end retouching portfolios (with rollovers) and let the photos load up, next without looking at the after (use your peripherals ) hover over the pic and if needed let it load up. Looking at the before, study it closely and figure out what
you would do if you where to fix it. I make an quick sketch of a generic head (if its a beuty retouch lol) and then jot down what needs to be done...not unlike what an art director might send to a retoucher showing what they want fixed. After I am satisfied with what I think should be done ( keep in mind of the skintones, blemishes, wrinkles, shadow density, eyes etc..) Now take your cursor off the mouse and look at the artists version...are you close? is there things you would fix that they didn't? Vice versa? Obviously there isn't any right way but this can get your close, also an art director might want to keep some blemishes or other parts, so their pic could be different in that way too.
I think its important to use quality artists portfolios for this, using it on a bad retouch won't help you, and can even send your skills in the wrong direction! I always look at the after and think does it look like it was retouched? (with the exception of looking perfect

)
Hope this helps some people, let me know what your thoughts are.