How can I make sure a string is not used anymore? e.g. cannotrestoreadminoredit | core_error

How can I make sure a string is not used anymore? e.g. cannotrestoreadminoredit | core_error

by Stamos Spiliotis -
Number of replies: 1
I tried to correct an english string from "... a editing teacher..." to "... an editing teacher..." (cannotrestoreadminoredit | core_error).
Then I noticed it is greylisted! That means it is not 100% sure it is abandoned.
How can I securely check and verify that it is not used 'anywhere', at least in the version I am interested in?