JTL The flicker (backlight) of the monitor can be seen even through the "pencil test."
This is especially noticeable on phones with OLED/Amoled screens. If the backlight flickers, it can be seen as "stripes" on the screen, on the phone's camera.
There is also pain in the eyes from the pulsation of the LED lamp (above your head). The "pencil test" works here, too. A more visual way, through the camera phone. I set 720p/240fps and change the shutter speed.
Answering questions, - Does my monitor have flickering? You need to go to the displayspecifications site and enter the model of your monitor. For example, Acer EK240YAbi (recently sold). We look at "Panel bit depth" = 8bit (6bit + FRC). Your eyes will hurt (due to FRC/pixel flickering). Although the backlight is (at the same time declared as) Flicker Free.
My eyes hurt from Intel iGPU too (even on the 8bit | Flicker Free monitor). I change the video card to Nvidia/AMD and the pain goes away. The same with HDMI cables (eyes hurt a lot). I change the cable to DisplayPort and the pains go away. On the monitor, Xiaomi X27G had to turn off "response time" (backlight blinking/black frame insert) from fast to normal.
The main thing is that you have a Flicker Free laptop screen. - Where to see? You need to go to the notebookcheck.com website and enter a laptop model. For example, the new MacBook Pro 14 2023 M3 Max (2023) has PWM = 15,000 (for $3200 it’s kind of cheap). When the same Apple-MacBook-Pro-13 (2020) has PWM = 113600 (7.5 times better). The higher the PWM, the less tired the eyes are.
Things are much worse with cheap laptops. Lenovo-IdeaPad-330-15ARR (2018) has PWM = 200 (200 in total, Karl!). Dell-Inspiron-15-3585 (2019) has PWM = 250. On older laptops (at <100% brightness), PWM was rarely >2000. Flicker Free laptops MSI Stealth 16 Studio (2023) or Lenovo Legion Pro 5 16 G8 (2023). The human eye does not see PWM above 25000. In any case, the decision "which laptop to buy?" depends on you.