A flute is essentially a series of precisely timed air leaks (the open tone holes) that must otherwise seal perfectly, so even one imperfectly sealing pad noticeably weakens tone and makes certain notes harder to produce cleanly. Beyond the light test, gently press each key fully closed one at a time and try to draw air through the open end; genuine resistance means a good seal, while easy airflow past a supposedly closed key indicates a leak. Old pads harden and shrink with age even without heavy use, so a flute that's been stored for many years often needs a full repad regardless of how little it was played, which is worth pricing into a used purchase from an estate sale or long-unused source.