TouchBarDemoApp is cool:

"Touch Bar Demo App allows you to use your macOS Touch Bar from an iPad (through USB connection) or on-screen by pressing the Fn-key. It shows the original Apple Touch Bar, which changes dynamically based on the app you're currently using. With this demo app, you can try out the Touch Bar on any Mac that does not have a physical Touch Bar."

I've put it on my iPad, and it works really well. Not only does it make developing Touch Bar stuff way easier, but you can also see what Apple is doing in their apps in a better light.

The simulator that comes with Xcode is OK, but not nearly as good as using something with a real touch interface (and actually closer to your keyboard!).