One of the indicators of poorly executed plagiarism is a discrepancy of style.
- formatting - for example, text inserted in a different font, size and colour
- spelling - for example sudden instances of British spelling in an otherwise American text - or vice versa.
- grammar - correct and complete sentences in an otherwise illiterate text
- fog - sudden clarity and precision in an otherwise muddy stream of prose
- logic - contradictory material pasted together with no acknowledgement
I did catch two identical submissions last year. Two scripts contained a particularly striking piece of idiocy, which I recognized on reading it for a second time. I had to wade back through the pile of already-marked scripts to find the first one, because they hadn't been quite stupid enough to submit the identical scripts consecutively.
Many of the worst cases of plagiarism are executed so poorly that they reveal the incompetence, ignorance and stupidity of the writer. So perhaps teachers should just fail such students for incompetence and ignorance, instead of trying to convict them of cheating.
At the other extreme, plagiarism can also be detected by an implausible level of sophistication. Andrew Bomford was told about an essay containing a word so erudite that the marker needed to look it up. It turned out that the student didn't know the word either. Fail.
The man who helps students to cheat (BBC Magazine, 12 May 2016)
Updated 13 May 2016