Anyone who considers persons of color to be inferior. The Proud Boys, Klan, et al are easy to identify, the more numerous, pervasive, and pernicious are the ones who would never classify themselves as such, but who have the power to control access to resources, education, legislative power etc and wield that power in favor of white people. Their reasoning is always subtle and seemingly logical. These are people who are on hiring committees who subconsciously look for flaws in the résumés of people with ethnic names; they are on admissions boards of prestigious universities where even a degree from one will open many doors, they vacation in exclusive places where their children see people of color only in subservient roles.
All these factors contribute. When white people in online discussions get frustrated because a demographic of color gets a lot of what they believe to be undeserved positive press, that’s an indicator of a White Supremacist. When white people rail about brown immigrants taking American jobs, and they fail to recognize that it’s the employers, usually white, that are hiring those undocumented brown people to avoid paying a fair wage, that’s an indicator of white supremacists.
For most of middle and upper class America, the seeds are well planted, and it takes constant, personal, vigilance to combat the subtle messages implanted in our conscious and subconscious awareness.
White Nationalists just try to justify it with geography.