A small windmill style air speed measurement device will give true airspeed although you still won't know how fast your plane could have gone without this extra drag on your plane. It would also have to be out of the propwash and away from any surface that would tend to accelerate the airflow around it, like the top of an airfoil.
Full scale pilots rely on pitot tube indicated air speed readings mostly because uncorrected IAS is a more useful bit of information than corrected air speed is. A plane will stall at approximately the same IAS regardless of temperature or altitude or humidity.