Test Your Focus
nytimes.com
nytimes.com
My own approach was to simply ignore anything blue; since my browser window is blue-framed, the Times background is black and white, and the UI elements are all blue, I simply blurred my vision and made note of the items that visually jumped out at me. I certainly didn't keep any of the blue items "in memory"; I only noted and remembered the two visually-distinctive ones.
I'm sure there's some kind of science behind this, but I feel like I just participated in a cheap parlor game.
Edit: Just to clarify, I scored 2/2, so this isn't just sour grapes about not getting a good score. ;)
Unfortunately, humans have extraordinary peripheral vision so even if you do tasks that destroy your concentration (in my case: selecting some tracks on spotify and reading the subject of a "new email bubble") you will still be able to sense whether the red bars rotated.
To really test concentration you should have some kind of continuous stream of information, so that you lose count the moment your mind drifts. That would probably give much better data. Then you can measure the average time before people lose concentration, and so forth.
I did only get one wrong (and knew it the second I clicked), but I think I could have done it faster without the extra mental processing to override my urge to click on the wrong spot.