How do you know that a record is really worn?

Hello All,

As the title says, how do you know when a particular disc is worn? Does the fidelity fall off and the sound becomes muddy? Does surface noise increase to the point where it becomes louder than the content?

I have acquired a quantity of vinyl that even after substantial cleaning just doesn't have the presence that I think it should. There are no visable scuffs and the grooves after cleaning look fine.

Thanks for your input.

Regards,

Jan