The present work presents a general approach for viscoelastic material characterization. The standard solid model and the fractional Zener model are taken into consideration. Model calibration is performed under the formalism of the inverse problems. Model validation and model selection are guided by the philosophy of the verification and validation field. Comparisons between measured data and model predictions take model uncertainties into account through Monte Carlo simulation analysis. This approach is used to calibrate models for two diglycidyl ether of bisphenol A (DGEBA) based epoxy systems modified by the incorporation of hydroxyl-terminated polybutadiene (HTPB), namely: DGEBA/HTPB blend (EPH) and DGEBA/HTPB blend modified with NCO groups (EPI). Three different types of tests are considered and a large group of test specimens are analyzed. The constitutive models are used to provide predictions for the loss factor of the materials as well as their confidence intervals. Predictions indicate that EPI presents better performance concerning mechanical energy dissipation than EPH which is highly correlated with the fact that EPI presents rubber particles distributed uniformly in nanometric scale within the epoxy matrix.