Site that does reviews of laptops, often includes PWM test done using oscilloscope with exact frequency (spoiler: most of them are pretty bad)
http://www.notebookcheck.net also sometimes says if a laptop uses PWM or not, but it doesn't show any data or say what method was used.
Here is the picture they use, for example, to show that the Lenovo T450s is PWM-free.
A good list and discussion
This has a familiar ring to it
... Anything that flickers gives me crazy migraines that go on for days...
Slacor Indeed it does.