That's fair, it is nice to be able to "see" results.
I think reading can be akin to meditation and improve focus - letting the body relax, checking out from the world a bit, and funneling attention into a story; if fiction, perhaps your creative side can light up a bit too.
One way to gauge progress might be getting to a point where you dont feel a strong pull of distractions or other things around you and can really enjoy or soak in what you read.
On another note, if you want to play with biofeedback, check out the Muse headband. It senses your brainwaves and gets louder when you are not concentrating, and softer when you are relaxed and focused. It comes with an app and such. I thought it was pretty fun to play with!
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