The linear viscoelastic behavior of various polydisperse linear polymers in the melt is used to predict their average molecular weights and polydispersity index. The method is based on simplified molecular dynamics and has been previously shown to enable a correct description of the dynamic moduli of polypropylenes from the knowledge of their molecular weight distribution (MWD). This so-called forward calculation only requires a few parameters, namely the scaling law for the zero-shear viscosity of narrow fractions eta(0) = f(M), the plateau modulus G(N)(0), and the value of the molecular weight between entanglements M-e. The main goal of the present work is to find a solution to the "inverse" problem. To avoid the problem of becoming ill-posed, the shape of the MWD has to be prescribed. Using the assumption of a typical logarithmic bell-shaped Wesslau MWD, the method has been proven to be successful for the recovery of the weight average molecular weight and of the polydispersity index of many linear polymers in a large range of molecular weights and polydispersity indices. Rough estimates of M-z have been obtained for well characterized polypropylenes by the use of a generalized exponential distribution (GEX). Attention has also been focused on some requirements for the frequency window which are necessary for reasonable accuracy of the values of the parameters M-w and I-p. It was found that, because of tube renewal and constraint release, three or four decades is generally sufficiently for a wide range which can be easily achieved with routine rheological dynamic measurements.