QUESTION: Is there one hard and fast way to always show instead of tell?
If you stay firmly in the viewpoint character's head and feel and see what she/he feels and sees, you will never tell rather than show.
Take the example of fear. If you are afraid, you don't just think to yourself, I am afraid. If you think that, you really aren’t that afraid.
Instead, you may feel a shiver run down the spine, your heart will pick up speed, your body could tremble, etc., etc.
If you write about what the fear feels like, that's showing. If you just say that the character is afraid, that's telling.
How do you get so firmly in a character's head?
Part of it is practice. Part of it is acting.
One of the most popular methods of learning acting these days is called The Method. The actor is supposed to immerse herself into the character so that she isn’t acting, she’s actually the person.
One variation of The Method is called Being in the Moment. I like that as a metaphor for what a writer does.
Put yourself in the moment of the scene.
When you are ready to write a scene, close your eyes and imagine where your viewpoint character is. What surrounds her? Are any of the objects around her of importance? How are they important? What are the sounds? The smells? Who else is around her? How does she feel about them? How will she physically react to them?
Now open your eyes and start typing.