This CLI Task lists all locallang labels, that can't be found in the given folders except the locallang file itself.
Call it like:
typo3/cli_dispatch.phpsh mktools_find_unused_locallang_labels --locallangFile=typo3conf/ext/myext/locallang.xml --searchFolders=fileadmin,typo3conf/ext/myotherext...
You have to provide the path to the locallang file and the commaseparated folders to search in recursively.
Of course, as always you have to create a Backend user named _cli_mktools_find_unused_locallang_labels.
This task is only available in TYPO3 8.7.