I'm not sure if you've read Mihály Csíkszentmihályi or not, but he argued that flow states are more likely when a task is more complex/challenging, and the person has a high level of appropriate skill that makes it possible (with substantial effort) to complete the task.

For me this often occurs while programming, sailing, or doing math- especially if I need to solve a new problem with those skills that will be especially challenging.

Once I'm in 'flow' it is a distinct mental experience - I am totally into it and lose any sense of time passing, or of needing to motivate myself until I am interrupted either by my own body, or by something external.

Cal Newport in Deep Work (his own word for flow work) defines "Deep Work" as anything that requires skills that would take at least a year to develop if a person was already generally educated, smart, and motivated.