Just asking the question, how to detect if a screen or monitor (LED backlighting probably) has Pulse Width Modulation to control brightness? This is known to cause eye fatigue.
Although you can use the various methods already in this thread.
Another way, is to use Google + the exact model number of the model. E.g. Written on the back of it.
There are some very good websites, which tell you (reviews), what if any PWM is used in the backlight. Failing that then the photodiode (maybe phototransistor, but LDR's are probably too slow to react, but might work) + a scope is probably good, as mentioned in earlier posts.
You can get monitors, such as some of the Benq ones, which don't use PWM (if I remember correctly).