I have become quite an expert at catching instances of academic dishonesty.
In fact, I've caught more plagiarists without the help of my school's detection program (SafeAssign, Turnitin, etc.) than with.
I say this not to brag, but to preface the following list: those sentences I'm mostly likely to say right before I catch you. I put this list in order of ascending frequency.
"Gee, that's not how the book put it." When a student, even a good student, knows more about the topic than the author he's supposed to be summarizing, analyzing, or assessing, I tend to assume that student has done a little outside "research." So I start doing a little "research" of my own.
"Gosh, this would have been a great paper for the other class this student is taking/took last semester/would have taken as a freshman." I do know what classes are being offered elsewhere in the school, you know. And sometimes, your other professors and I chat about, like, what we're teaching. Recycling paper is good and will lead to a cleaner and better future for all; recycling papers can only lead to your downfall.
"That's . . . not . . . what I assigned." Sometimes, an essay that doesn't address the assignment but is in other ways a sound effort is just a bad essay. Other times, it's a bad deal--that purchased essay didn't exactly do it's job. And the funny thing? About two sentences into our conversation about your paper (and we will have one), I can tell which of these it is.
"This is a rather astounding improvement over the last paper." This sentence almost never, in itself, provokes The Conversation. I've actually had students who work on their writing and improve it over the course of the semester. (No, really! Some people do that. Have you considered that as an alternative to . . . no? Well, it was worth a try.) But it almost always provokes a little one-on-one time with the search engine.
"Wow, this is a grammatically correct sentence." You know who you are and why this makes me start googling. This corollary to the previous (both falling under the "There's no way that student produced this paper" category heading) is distinguished primarily by the suspect sentence's isolation from its surroundings. That pearl of great price--grammar--shines all the brighter when it appears in the midst of the compost bin that is your usual writing. I always google the pearl. And I usually find that it has an original owner that is not you.
"Darn that thesaurus function on Word." This one is kind of fun, actually. When I reconstruct the sentence that would have existed before your thesaurus use rendered it nonsensical, I can figure out whether you're 1) a bad writer or 2) a bad writer and a plagiarist. If the probable ur-sentence of the tortured sentence is itself confused and mangled, that's a sure sign that it actually originated with you. (I never attempt to reconstruct the mental pattern that produced such a sentence. I prefer to live in a land of logic and reason.) If the reconstructed sentence is logical, grammatical, and germane to the essay, I go a-hunting.
And, the final and most frequent precursor to your visit with the academic dean:
"Oh, look! Wiki!" Nowadays, I read the relevant Wiki page(s) right before I start grading. It just makes the whole process faster.
Perhaps this list will make you reconsider your attempt to skirt the requirements of your course. Or perhaps it will simply make you a better plagiarizer. Either way, I will consider it a success. The former is better for your soul, which is, of course, more important than your intellect. But the latter is not lacking in a kind of scholarly merit--the skills of a successful (as opposed to lucky) plagiarist are genuinely intellectual skills that may serve you later in life.