How to detect PWM on a screen for brightness

What methods guys do you use to detect that a screen uses PWM to control the backlight brightness?

 

I think I've found an easy way to detect screens that use PWM at low frequencies.
In my case I'm trying to detect PWM on my Sony Vaio VPCZ13, and the method used is a smartphone camera in manual mode. In this case, I open the camera of my LG G4, set it to manual, and do the following steps:

• Set laptop screen brightness something in the middle, like 30 or 40%.
• Set the camera shutter speed on the smartphone to the highest level, in my case the LG G4 has the highest value of 30 seconds
• Then decrease the shutter speed step by step until you see flickering on the camera (if it happens to happen). For the Vaio Z1, with a shutter speed of 1/8s nothing is noticeable, but at the next step 1/15s you already start noticing a little flickering depending on the colors being shown on the screen. At a speed of 1/125s the flickering is so high that you couldn't record a video comfortably. You don't need to take a picture, just open the camera application and point.
  
• Keep increasing the shutter speed until you notice flickering, up to the highest shutter speed. The LG G4 has a fastest shuttering speed of 1/6000 but you would need a lot of light to see anything.
• In case the stock camera app of your smartphone doesn't allow manual settings, try updating the operating system or use third party apps like Camera ZOOM FX Premium.

In case you aren't able to detect it may be due to several reasons:

1. The laptop/screen brightness is at 100% (some screens at the highest level don't flicker because the current flow is continuous instead of PWM).
2. The camera shutter speed is not within the adequate range to detect flickering.
3. The PWM happens at a very high rate so can't be detected easily.
4. The screen doesn't have PWM at all (like with the Asus ROM G751JY gaming laptop, or the BenQ GW2760HS Flicker Free LED Monitor).

 

Found an amazing blog explaining in depth more about the PWM and screen flickering:

Flicker in the display due to PWM - irresponsibility of the manufacturer or technological inevitability?

Example of youtube video showing PWM in Sony Vaio: