To determine first if A Square is a Rhombus.
Let us define a rhombus.
1) Has all four sides equal.
2) Has opposite sides parallel.
3) The diagonals bisect the angles and bisect each other at 90 degrees.
A Square also displays all of these properties.
So a Square is ALWAYS a Rhombus.
But a Rhombus is a not always a Square, except that Rhombus is a Square.