This is something that I constantly employ in my arsenal of learning tools. You know how you sometimes gaze at a picture just to figure out how it was lit? Well, for retouching I like to gaze at the histogram to figure out how it was retouched.
You can learn so much from an image by looking at the histogram. Assuming you know how to read it. Even I'm still learning. But as with gazing at an existing image for lighting nuances, we're backsolving for the original adjustments when reading the histogram.
As and example, channels that are divergent in their representation (i.e. not overlapping) usually indicate to me cross-processing or that they've been adjusted with gradient maps. Differences in curves throughout the histogram also allow me to infer certain curves/contrast/levels adjustments.
Now, it's not an exact science. We know many features of Photoshop can accomplish the same result. Furthermore, the combination of several adjustments might result in what seems like a single unique adjustment. Backsolving isn't an exact science. Just like backsolving a picture for lighting, you'll never know exactly how it was lit... but you might come close. What you know is what you see. It's like forensic science except that reading the histogram is much less exact.
But RGB and LAB tell stories (though I don't read LAB). Stories that can provide us with a lot of insight into what "happened". I'm still ever curious about how pictures came to "life". In that respect it's exactly opposite of forensic science when so often you're trying to figure out how someone died :)
So save a kitten! Read a histogram.
By the way, the histogram above came from a crop of this.
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