How do you determine if a polygon is a regular polygon?

This is a tautological question that does not have a proper answer. A regular polygon is one which has all its sides of equal length and all its interior angles of equal measure.

A regular polygon is a polygon whose sides are equal length and whose angles are all the same value. A non regular polygon is just the opposite of a regular polygon.

No, regular polygons are always convex and are shapes constructed using straight lines.concave polygons are irregular.

A rhombus has four sides of equal lengths. So it is an equilateral polygon. But it is not an equiangular. It's angles are not equal in measure.

A rectangle, for example.