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If you hear grinding metal, it is already too late and you will as a minimum get to turn the rotor or even replace it.
The pads on my 94 F250 do not have any wear indicators so when they are gone, they are gone.
Some cars have a metal spring feeler that chirps the last maybe 5,000 miles. Others have rivets which will screech when you get too low. My Ford just has a nice metal backing plate which grinds up rotors just dandy.
Been there done that.
A visual inspection is what you want and DO look at the pads behind the rotor, that is where mine wore out first and ate the rotor.