Some of your authors will understand the science of etymology. You will get to know those authors if you run across them. They are fabulously rare.
Others will take the trouble to look up etymologies in a dictionary. These authors are only slightly less rare.
Most writers who give you the derivation of a word are giving you an assertion they heard from a pedantic teacher in junior high school and never thought to question. Sometimes those etymologies may be right—generally when they are etymologies of complicated technical terms, which by their artificially constructed nature have very simple derivations. If your author tells you that hydrophilic comes from Greek words meaning water-loving, then her junior-high-school teacher was correct.
But these junior-high-school teachers usually go horribly wrong when they tell you the derivations of ordinary English words. And we can learn something about etymology just by seeing where they go horribly wrong. Here is Christopher Booker (whose fascinating brand of crankiness is exactly the sort of environment in which we are likely to find false etymologies) recalling what he must have learned in grammar school:
They [viz., ancient monumental structures] stood for that sense of “wholeness” which we derive from the Greek holos, which also gives us “holy” and “holiness.”
With admirable efficiency, Mr. Booker crams three false etymologies into one sentence. Wrong, wrong, and wrong. Dr. Boli could explain why the derivations are wrong, but anyone can look them up in the dictionary—Wiktionary is a good place to start. It does happen that “whole” and “holy” are related Germanic words (and both related to “health,” which would have given Mr. Booker much to think about if he had taken the trouble to open that dictionary), so the point could have been made without dragging in the unrelated though coincidentally similar Greek word.
But we see here one of the most common patterns in mistaken etymologies. Greek is ancient; English is modern; therefore any English term must have come from Greek, because Greek is older. The most superficial knowledge of history ought to be sufficient to dispel the notion that common, ordinary English words would have come from Greek: English is a Germanic language, and why would ancient Germanic tribes have imported their basic vocabulary from the Aegean? But your pedantic junior-high-school teacher never thought of that.
Another example comes a page later in the same book.
The Greek and Latin words for “god,” theos and deus, derive from the same Sanskrit root dyaus from which we get the word “day.”
Wrong, wrong, and wrong again. Surely Mr. Booker knows enough history that he does not seriously believe that speakers of Sanskrit, the highly developed literary language of India, moved west and became Greeks, Italians, and Cockneys. This is one Dr. Boli didn’t have to look up: it is a general principle that almost any derivation of a word in a European language from Sanskrit is wrong unless it refers to a specifically Indian phenomenon. Nevertheless, Dr. Boli did look up the words, because the dictionary is right in front of him, and it seems only fair to confirm his assertions before passing them on to the rest of the world as God’s own truth. Wiktionary informs us that, “despite its superficial similarity in form and meaning,” deus is “not related to Ancient Greek θεός.” The etymology of day is uncertain; at most it could be a distant cognate of the Sanskrit term, but, no, the Anglo-Saxons did not learn their word for “day” from the Indians.
These wrong etymologies are easy to catch, but no copy editor bothered to catch them. They came from Mr. Booker’s 2004 book The Seven Basic Plots, which Dr. Boli has been reading the past few days, and often wondering whether it went through a copy editor at all. Supposedly the author spent 34 years on the book, but few of those arduous years were spent in front of a dictionary or encyclopedia. And where was the copy editor? “Pharaoh” is a hard word to spell, perhaps, but a copy editor should know it’s a hard word and look it up, even if nothing about the spelling “Phaoroah” triggers alarms. The seven plagues of Egypt is giving Mr. “Phaoroah” an undeserved 30% discount. Dr. Boli is mystified how so obviously well-read a writer can write “thou cans’t,” as if it’s a contraction of some sort, every time he quotes “thou canst” from the Bible or Shakespeare—and even more mystified that a copy editor could let it pass. Mr. Booker himself clearly is quoting many of his sources from memory, and even when he gets the words right he often misremembers the context. Where was the copy editor?
It is interesting to note, therefore, that most of Mr. Booker’s later career was marked by his repeated insistence that he knew science better than all the world’s scientists, because he had sat and thought about it for a while, and presumably they had spent their lives playing badminton or watching soap operas and picked up their degrees at a rummage sale. Dr. Boli has not read any of those later attacks on science, but it would not surprise him to find that they were full of the same kind of thing: it must be true because it stands to reason, and I don’t have to look it up because I already know.
But that is a digression from the main point here, which is that, if you are a copy editor, you should assume that every etymology your author gives you is wrong, and look them all up. Wiktionary is a very good tool. The Oxford English Dictionary is unlikely to steer you wrong. If, after half a dozen etymologies, you find that your author has always been right so far, you may begin to relax your vigilance. But whenever a writer tells you where a word came from, your default assumption should be that the derivation is incorrect.