a rhombus is a parallelogram, namely, is a polygon with four sides and two pairs of parallel sides.
the one distinction of a rhombus with other parallelograms is that, a rhombus has all equal sides, all its four sides are the same length, so, if you already know the vertices are for a parallelogram, then is a given two pairs of sides are parallel.
so... you simply need to check their length, making sure they're all equal, using the distance formula.
