For those of us in Humanities areas who teach/assess writing, plagiarism is very easy to spot a mile away. The writer's "voice" shifts, syntax isn't consistent, etc. In a follow-up conversation with the student, it becomes obvious that they can't articulate the ideas they wrote about. When I started teaching, that was enough "proof" and departments would back up their staff's assessments. Now it's a lot harder, with most Univs requiring documented proof--tracking the sources stolen from--so services like Turnitin are needed. Otherwise, students get their parents to complain above your head and it becomes a s***storm. Never got a call from an embassy, thank God!, but often had to present the case to visa sponsors. Sigh.