You can wait around at likely streets after a project ends, or you can check the map: a greyed-out street will appear. That's the street which is being built. If you're not sure which street houses the vendor to talk to about the project, join the "New Streets" group, and ask in the group chat window. Group members will also tell you what's needed at specific projects, so you can bring appropriate supplies.
This is also the best way to finish the dreaded ingot-donation quest: hang out near the projects, ingots in hand, or get to them quickly when the call goes out in the New Streets channel.
Also, the loading screen which appears when you join a street will say "Project Available!" if there is one.