I mean programmatically be able to determine if a subtoken is “part” of a word or a word itself, regardless of the tokenizer. So given this:
... ('die', 17), ('Er', 17), ('##mitt', -100), ('##ler', -100) ...

A function that uses either the subtoken or the id, to return True/False if it is part of a word or a word itself …
tokenizer.is_token_part('die') #    => False
tokenizer.is_token_part('Er') #     => False
tokenizer.is_token_part('##mitt') # => True
tokenizer.is_token_part('##ler') #  => True

And again, this would work across all tokenizers.