To determine if they are employees we look for ways the Uber/driver relationship is similar and different to other businesses using the employee model.

To determine if they are contractors we look for ways the user/driver relationship is different from other businesses using the employee model.

If no other businesses allow employees to set their number of hours with such flexibility then that is evidence the relationship doesn't fall under the employee model.

This need not be all or nothing but it is (obviously) a factor.