Here (below) is a simple reasonableness formula that relates to the relationship standard of general respectfulness. It is based on a simple ownership test. It is rooted in the idea that a person's true character is not so much revealed in momentary departures from ideal behavioral standards, but rather, in how they deal with the opportunity that presents itself when a moment of their behavioral issue is pointed out.
No one is perfect. Everyone will fall short of the ideals presented in these relationship standards from time to time, even if they espouse and advocate them. Expecting perfection is a reasonableness issue, itself. However, moments of gratuitous disrespect should not be overlooked when they give rise to reasonable questions about the underlying attitudes towards you that they may reveal.
When you are treated with unwarranted disrespect, it is sometimes obvious, given the context of the moment and your experience with the individual, that the particular disrespect which seems apparent is more about some other factor troubling the individual, than about how they truly feel towards you. Perhaps they are hungry, stubbed their toe, or misunderstood something you said or did. Or, perhaps not.
When it isn't clear that the moment of disrespect shouldn't be taken seriously as representative of how they truly feel about you, it becomes fair to ask the questions necessary to find out. These questions involve a reasonable confrontation of (complaint about) the moment of disrespect. You reflect back to the individual what they just did or said, or how they did it or said it, and point out that their behavior involved disrespect that you believe to be unwarranted.
The immediate and most relevant point of the confrontation of the behavior (complaint) is to ask if they agree that the behavior was unwarranted. This gives the person the opportunity to either own the behavior and apologize for it, or, in the alternative, either deny or attempt to justify the behavior, avoid the discussion, or gas light the complaint.
If the individual owns the behavior, its illegitimacy, and apologizes for it, you have your answer. Generally, this is the end of the matter, because it is all you really needed to know. The momentary instance of disrespectfulness can be overlooked, unless it is chronically repeated and the ownership and apology seem insincere.
In healthy relationships such non-representative moments of behavioral imperfections are resolved within seconds to the mutual satisfaction of both parties. There is no meaningful disruption to the relationship. The moment of inquiry is appreciated, understood, and treated with the respect it deserves. In return, the behavior at issue is given a pass, because it deserves a pass. That is the end of the matter and the health of the relationship was enhanced by the fact that the moment got the attention, but no more of the attention than it deserved.
If, on the other hand, the individual denies the behavior, avoids the discussion, or gas lights the complaint, a very serious and telling revelation about their character, and about how they regard you has been revealed. You now have some very serious choices to make.
You cannot, for your own mental health and well-being, allow yourself to be continually treated with disrespect by anyone. Abandonment of the relationship, altogether, might be warranted. In some cases Grey Ribbon Management techniques become relevant and useful.