Mark, this is a very good question!
It is usually easy to spot the bad ones and the really good ones. But then there is a host of seemingly (similar) ok shots. To pick the best out of that bunch is the hard part.
Practice helps, an I find it very helpful to go through the shots together with my wife as well. Sometimes you dan't have that luxury though. So any help would be nice
I'm afraid though that the answer is: practice, practice, practice