One litmus test that a former coworker applied was "how resource constrained is the application"?

I think conceptually this is a better test, more so than say if you have access to a raw pointer - because more important parts of how to handle core issues like scheduling, storage management, concurrency, parallelism and distributed computation manifest themselves in many forms.

I started my career doing C programming in a OS / kernel environment. In retrospect a lot of pieces of applications I've worked on are what I think of as systems, and many parts of kernels are more applications than systems.