Unfortunately it is subjective, I've had to return Motorola phones that did not have PWM as they still gave me eyestrain / headache / migraine symptoms.
It is time consuming and expensive but you will have to test devices, ideally at home as you cannot really test fully in a store as you cannot be there for several hours and the lighting in the store may affect testing / how you percieve the screen. Before you pay for any device be sure that you can return it first, this differs depending on local laws etc.
I would suggest phones that do not have PWM as a start, so you cannot use any phone that has OLED / AMOLED screen as they will use PWM to flicker the backlight. Look for LCD screen phones, use sites such as notebookcheck to see if they have PWM althought they are sometimes incorrect with their testing and it may depend on the region where they get their phone to test.
Or try buying older handsets on ebay etc as they are cheaper, but you will not be able to return them usually.