Check the copyright page it should be printed on the bottom of the page. If not check the publishing dates, this isn't always accurate as popular books can be reprinted in the same year.
You would have to match the Copyright date on the inside with the known publishing date. You would have to validate it against a listing of the first edition dates.
Sketches by Boz was the first serial to appear in a magazine, but The Posthumous Papers of the Pickwick Club was the first to appear as a novel, both in 1836.
In 1836, Dickens published his first book, Sketches by Boz, a collection of articles that he had written for Monthly Magazine and the London Evening Chronicle.