You know you're developing an internal voice for writing when you find yourself avoiding or approaching other authors on the basis of how they'll affect your own voice during a writing session.
I'm reading an author right now with whom I've got a push-pull relationship. Her voice and in turn her ethos or stance can become too influential on some days, like a fascinating but strong-willed companion. But I have also become strong willed so it's a balancing act, deciding when to use her work to jump-start my day and when she is crowding me out. I need a sensor for that impact.
Sometimes picking up a particular writer serves to "jump start" my writing day. Just a few sentences or lines, and my own writing voice gets fired up and ready to work. Invention begins as a desire to join the conversation started by the other writer. It really doesn't matter what the author is writing about: more style/approach and less content/subject/genre.
It's frequently helpful and even inspiring to get a draft done that way, imitating and picking up on another writer's voice. You can always go back and change your style at a later moment, a later draft. From this method, you may see new angles and find material. Adopting a voice can produce an interesting base coat and challenge a writer to think differently (aka "critical thinking"). Sometimes though I need to enforce a complete break from another writer's strong voice early on. It's all about assessing and accepting what's really happening.