I was a TA for a chemistry professor to whom I brought evidence of falsifying data. She shook her head with a small smile and brushed my anger off. She said "I know they're cheating, but if you can cheat well enough to accurately falsify the data, then I have taught you what you need to know from the experiment." She was a pragmatist.

Another professor of mine, a wily old Thermo teacher, had a student come up to him with a test. The student was insisting that the professor had marked his correct answer as wrong (this was after the prof had gone over the answers). With a twinkle in his eye, the professor said, "No problem, I'll fix it, but when I look at this paper under magnification, I will find that the ink from the pen is on top of the pencil graphite, not the other way around, right?" The student had a moment of panic and quickly took his test back and slunk away. Called out!

Another student, when I was a TA, spent all night writing tons of formulae on a desk in an old test hall, planning on getting the seat early and copying from it. We noticed it as we walked in, and decided to stake the seat out. However, a non-cheating and studious student sat in the desk because she got there earlier than the cheater. We waited for the test to start, and the student who wrote the formulae stumbled in later, Red Bull in hand. He asked her to swap desks with her and we nailed him.

I was excellent at catching cheaters, but my methods of reprimand weren't well liked. In a hall of 300 students, as a TA for Physics II, I found a student cheating, and snatched his test, ripped it up, and then broke his pencil and yelled at him to get out. I got remanded for my draconian punishments, especially since the university had some backwards cheating policy, where a fellow student had to out the cheater, or it wasn't enforceable.

Many teachers are excellent at catching it. Students aren't as clever as they think.