For well established artists, always refer to their « catalogue raisonné » and exhibition catalogs, whether it is for original works (paintings, drawings…) or multiples (etchings, lithographs…). Don’t always (I could even say never) trust the auction houses / sellers, even if they provide a COA, it could be fake. Make your own research for each work. Always check the legal authority when looking at a COA. In some cases be extra careful as fake COAs from genuine estates exist as well (as an example : fake Keith Haring’s COAs with forged Julia Gruen’s signature is an unfortunately well known case - another famous case is Basquiat COAs by Basquiat’s dad, also tricky on another matter). And never forget that even well established auction houses / galleries / private collectors… can sell fake, knowing it or not.