I made a terrible mistake with my grand loop of Brooklyn. It actually went into Queens, the horror. The section through Highland Park is actually across the border. Also, when I did the ride I used Wyckoff instead of Knickerbocker as on my route map, and one side of Wyckoff is also in Queens for a stretch.

You used to be able to distinguish between Brooklyn and Queens by the color of the street signs but these were replaced a while ago. The other way to tell is by the house numbering. Queens uses a dash (39-01) whereas Brooklyn does not (3901) , but at the border even this isn't followed closely.