Carey Reams adopted "see everything you look at" because so many of his agricultural clients were shortsighted. He wanted to bring them along on a long training process. It is very difficult to fully explain what he taught over the years, but a few examples may help. It also helps to think of him as a Sherlock Holmes type---seeing many clues that most overlook.
Here is an example: many people look at crops and don't notice that some plants are ignored by pests and others are greedily attacked. When you test the Brix and find that the pest-free crops are high Brix and the damaged crops are low Brix, you can't be so easily fooled the next time. You will "see" what you are looking at.
Here is another example: one field will dry out a day after a scanty rain and another field across the road will remain moist. Plants in the latter field will thrive even when the extension agent is claiming the county is in "a drought". That has to do with the carbon content (think organic matter) and you again have to "see" what you are looking at.
A third example has to do with people. Once you understand that the narrow pointy face of a kid who needs braces is due to a poorly-mineralized diet, you can never look at a kid's face again without "seeing what you are looking at." This one is not strictly Reams, as Weston A. Price, and William Albrecht also addressed the phenomenon.
As a person practices more "seeing," they start noticing things they never realized. Even medical doctors are trained to see conditions lying ahead that the layman will not observe. A case in point is how a good doctor (yes, there are many) can know years in advance that someone will one day suffer Huntington's chorea.
I guess I can close by saying "seeing" is an art as much as a science.