The first vertical bar (on the left) is black and can't be seen against the background
The second vertical bar is only just visible as a dark gray.
The third vertical bar is a lighter gray and clearly visible.
The white box is an acceptable level of true white.
Obviously, you either need an external monitor, or a fairly calibration-capable computer monitor.
What I do, is to put a small bit of white paper over the white bar, to see if it's "blooming" or not. That'll help a lot. Easier, too.
