Don't ask me how I know, because I don't know how, but copied work in a student's paper just jumps off the page. I usually detect it in under two seconds, and not because I'm looking for it, it just looks wrong immediately.
At first, when I was looking for the original source that a student copied from, I would guess what the most unique combination of words was, picking two or three multi-syllabic scientific-sounding words, and then do an internet search for them.
But I noticed a curious thing. In most cases, those unique words I was looking for often weren't particularly unique. But what did tend to be very unique was the "packaging" of banal words around them.
I've been shocked to find that, given about 5-6 words in sequence, it is almost always possible to find the one unique occurrence in the history of the English language that those particular words occurred in that particular order.
So now, I just type in the first 5-6 words of a sentence, no matter how banal and mundane the sentence structure sounds, and it almost always uniquely identifies the source. Neat trick, eh?
Try it at home. Grab a few words from anywhere nearby. Type them into your favorite web search. Watch the magic unfold.
See how many words you can get rid of, especially the words that sound most unique, and still have that one source pop up to the top of your list. (Hint: using quotes around the words to hold them in their original order makes it a lot easier.)
Somebody needs to write a paper about this phenomenon.