The full, original title of Joseph Andrews offers a window into the thematic points in this "epic poem in prose," as Fielding describes it himself. The title is: History of the Adventures of Joseph Andrews, and of His Friend Mr. Abraham Adams, Written in Imitation of the Manner of Cervantes, Author of Don Quixote.
The first giveaway of it being a Bildungsroman, or a coming-of-age novel, is that the reader witnesses the progress of the main character, an unlikely hero such as Joseph, going against the odds in a series of different adventures, events, and circumstances starting from the very beginning of his life as it is evidenced in the title of chapter 2, volume 1:
Of Mr. Joseph Andrews, his birth, parentage, education, and great endowments, with a word or two concerning ancestors
Further evidence of Joseph Andrews being a Bildungsroman is that Fielding offers that it is written "in imitation of the manners of Cervantes." If you read Don Quixote, you can appreciate the picaresque tones intertwined in a very dynamic narrative that moves forward along with the characters' wide range of actions, reactions and, most importantly, their emotional growth.
The reader's emotions change and grow along with those of the characters. This process of maturation is what makes the novel a coming-of-age story where we get to witness the character, literally, transform into something greater than what it is in the beginning.
Therefore, when Fielding names this an "epic poem in prose," he is not exaggerating. This is literally a story of epic proportions dealing with the development of a character going through changes that lead to its maturity and growth. This is the essence of all coming-of-age novels.