In my experience, the big problem really isn't that students so often seem unaware of whether their own work contains plagiarism. The problem is that they so rarely ask.
Of course I want them to ask ME about it, but the first problem is that they often don't even ask themselves.
The central question in this poster cuts right to the heart of the matter and tells students when they should at least WONDER whether they are plagiarizing:
"Is it possible that someone could look at your work and think that someone else's words, ideas, information and/or organization were yours?"
If there is any doubt about this, the student is directed to bring their work to the teacher and update it until the both the student and teacher are completely sure that the student has successfully avoided plagiarizing.
If instead or in addition you would like this in a no-frills printable form to give to your students, you can find it here: