First of all, I would argue that this question sets up something of a false issue. That is, it asks how a parenting expert can overcome their biases. A parenting expert would deny that their views constitute biases. They would argue that their views of how parenting should be done come from scientific research on the subject. A parenting expert might say something like “I do not have biases. I have professional, expert opinions.” Therefore, the expert would not feel the need to “overcome” their views.
If I were a parenting expert, there would be two ways in which I would judge a person’s parenting style. First, I would judge it based on how closely it conformed to what my study told me parenting should look like. If I believed in an authoritative parenting style, for example, I would watch to see if the parents set out firm lines that their children could not cross. If they did not, I would conclude that they were too permissive. I would look to see if they were willing to engage with their children in dialogue about their expectations and rules. If they were not willing to do so and, instead, adopted a “do as I say, or else” stance, I would conclude that they were too authoritarian.
A second way that I would judge their parenting style would be through observing and interacting with their children. If the children seemed to have a good mix of being independent but well-behaved, I would conclude that the parents were generally doing a good job. If the children did not show fear of punishment for making mistakes but, at the same time, knew that there were boundaries they should not cross, I would conclude the parents had done well.
As to bias, other than what I said in the first paragraph, I would try to make sure that my biases regarding the parents did not distract me. I would try to do this by looking at the behaviors that the parents exhibit, not my feelings about the way they look or their religious beliefs or anything else that is extraneous to their parenting style.
Thus, I would try to base my judgments on observable behaviors. I would judge parenting styles based on my observations of the parents’ behaviors and those of the children. I would try to focus only on behavior, not on anything else that might cloud my judgment.