A method and apparatus for characterizing whether a head of a data storage device exhibits excessive performance degradation. The device includes a data transducing head adjacent a recording medium, and read channel circuitry with a variable gain amplifier (VGA) and gain control block. The gain control block supplies VGA gain control values to the VGA to nominally maintain amplitudes of readback signals obtained from the head within a selected range suitable for remaining portions of the read channel circuitry. A baseline VGA gain control value is first obtained, after which the read channel is parametrically adapted to optimize read error performance. An adaptive VGA gain control value is thereafter obtained, and head degradation is determined in relation to the magnitude of the adaptive VGA gain control value as well as in relation to a difference between the VGA gain control value and the baseline value.