[rowe]

You can use an Opple Light Master to measure the PWM. But as others said the only thing that really matters for an individual is whether the PWM on the device affects you or not. For this reason I've never bothered buying one as I quickly tell whether the PWM is affecting me or not.

The only readers I've ever known marketed as DC-dimming are Hisense. But even though their lighting is probably the lowest risk for uncomfortable PWM, even DC-dimming lighting sometimes uses PWM between steps.

Ereaders that don't have negative PWM affects for me:
Hisense, old Boox (Poke Pro, Nova 2/3), various Kindles

E-ink devices whose PWM lighting is unusably uncomfortable for me:
Dasung monitors, newer Boox (Palma, Page, Tab Ultra C, Mira)

This will vary across users as different people have different PWM sensitivities.

Edit: Boox say their lighting is DC-dimming so maybe it's another issue for me on their devices, or I'm also sensitive to some between DC steps PWM