Multiple-beam Fizeau interferometry is an accurate, simple, and nondestructive technique in the field of fiber research. Inserting the fiber into the liquid silvered wedge causes the fringes to shift across the fiber region with respect to the fringes at the liquid region. Fringe shift is a function in the geometry of the different regions of the fiber and the refractive index profile of the fiber. In a previous paper (Applied Optics, Vol. 50, No. 31, (2011)) the theory for the interference pattern formation was developed. In this contribution, we extend the theory to include graded index double-clad fibers. In addition, we discuss the experimental aspects of the measuring system.