A method of determining that a thermoplastic was insufficiently dried before forming into a molded plastic part. The method comprises preparing a mixture of a thermoplastic polymer and an off-gassing compound, wherein the off-gassing compound is selected to release all water of hydration in response to target drying conditions specified for the thermoplastic polymer. The mixture is dried at actual drying conditions, and then heated to reduce the viscosity of the thermoplastic polymer and allow the mixture to flow. The mixture is then made to flow into a mold to form a plastic part at a molding temperature, wherein the molding temperature will cause any remaining water of hydration in the off-gas sing compound to off-gas and form surface irregularities in the plastic part. Non-limiting examples of the off-gassing compound include hydrated metal halides and ionic hydrates.