Anthony W. Shipps wrote a book called The Quote Sleuth, which contains tips on “the tools and methods used in the identification of the sources of quotations” and is really the definitive work for anyone serious about tracking down the correct attributions for all kinds of sayings. The book is an exhaustive how-to guide for tracers of lost quotations, and Shipps includes lots of examples of puzzling quotations and how he managed, with a lot of persistence and ingenuity, to finally pinpoint their original sources. Towards the end of the book, Shipps lists some axioms for would-be quote sleuths that occasionally approach the aphoristic and which I hereby excerpt:

Time spent looking for quotations is never wasted.

The Oxford English Dictionary is a book of quotations.

You should get ready for quotation problems long before they happen.

Solutions that you cannot find sometimes find you.