Late 19th and early 20th century adoptions in the US

My great-grandmother, Zula May, was adopted by the Goodrich family in Minnesota when she was a few years old. All of my family records indicate that she was born in New York City, but I don't know how much faith to place in that information.

Can anyone provide me with resources for researching adoptions from that era? I'd love to find any clues as to her actual heritage.