There are a number of ways in which to answer this.
First, a parenting expert would have to keep in mind that culture and socioeconomic status do not change what good parenting looks like. In other words, a parenting expert would need to make sure that they do not give in to the ideas of relativism. They need to make sure that they do not simply excuse bad parenting because it is the norm for people from a given culture or socioeconomic status.
Second, a parenting expert might put somewhat more emphasis on looking at the child and less on looking at the parent when analyzing people from a different culture or socioeconomic status. For example, if an expert only looks at the parents from a culture that does not believe in expressing affection, they might conclude that the parents are doing a bad job. The parents might not show the sort of outward love and support for their children that Western parents do. The expert might need to look at the child to see how well adjusted he or she is. It may be that the child knows that their parents love and support them implicitly. This will show up in their behavior even if it does not show in what the parents do.
Finally, and perhaps most importantly, the expert simply needs to be aware of the fact that the parents are from a different culture or socioeconomic status. The expert needs to be aware of any ways in which people from the subjects’ culture or status tend to parent differently from people of the mainstream. The expert needs to be sure that they are aware of any biases that they might have towards people like the parents. It is not possible to completely step outside one’s own prejudices and preconceptions. However, it is possible to be aware of them and to try to adjust for them. Therefore, the expert must be aware of who the parents are and of how the parents’ culture or status might color the expert’s view of their parenting. That is all that really can be done since human beings are unable to completely see through the eyes of another.