The same as any other position player, by their ability. The standards are hitting for average, hitting for power, and defensive fielding and throwing. In addition, catchers are rated on their ability to call the right pitches, to know their pitching staffs, and to get inside batters' heads.
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Good Catchers are usually rated according to their game calling ability as well as their ability to throw runners out if they are attempting to steal a base however Catchers are also rated according to their offensive skills such as if they can hit for average or if they're a power threat such as being a Homerun Hitter or even a Catcher that is a Clutch Hitter that has a career habit of driving in runs when their team needs the runs.