Twine's documentation says that what Test mode does is dependent on the story format. With SugarCube 2, what is different between playing and testing? Is there debug printing anywhere (or at least a flag that I can check in my printing)? I haven't found anything in the documentation...
Currently, there is no difference between Twine 2's Test and Play views in SugarCube (1 or 2). Debugging has traditionally been handled via the global SugarCube object, which you can inspect via the developer console.
If SugarCube can somehow distinguish itself being run in Test vs Play mode, could you add a user accessible variable (ex. a story variable "test" evaluates to true when in test mode and otherwise false)?
Unfortunately this feature seems to be broken in Twine 2.0.10 when in Test/Play mode, or at least the Window 64bit release is.
Usage is easy enough: