### Theory:

Using the signs of a rhombus, one can determine whether a given quadrilateral or parallelogram is a rhombus.
1. If the diagonals of a parallelogram are perpendicular, then this parallelogram is a rhombus.
2. If two adjacent sides of a parallelogram are equal, then this parallelogram is a rhombus.
3. If the diagonals of a parallelogram are the bisectors of its angles, then this parallelogram is a rhombus.
4. If all sides of a quadrilateral are equal, then this quadrilateral is a rhombus.