We have the technology to tell the difference between mercury coming from fish verses metals coming from your teeth or other sources. Labs can separate methyl mercury from inorganic mercury and measure each one directly. This way we can evaluate not only your mercury levels but also your exposure source.
One problem with eating fish (which I believe fish with fins and scales is the healthiest food on the planet) is the heavy metals, especially mercury. This can be especially harmful to pregnant women and small children.