How can the teacher find out whether the grade has been signed by parent?

At first, display the signed grades via the "Show" buuton. Tick the "Mark signed grades" and "Show also seen by student account" in the "Show" tab.

If the grade was signed by the parent, a green sign appears. If the grade was seen by student, a grey sign appears.

To sign grades, the parents have to login to their EduPage parent account. When they login to the student account, the teacher is unable to determine if the grade was seen by the student or by the parent.
Teacher - parents' accounts

For more information, please check:
How to show/hide grade details: comments, signed grades, date of grades or average?