Within the US, what steps could be taken to try to prevent racism in the police?
For example, there is no strong 'test' for racism that can't be double-guessed. Even then, there is the potential that some racist high ranking officers would be more lenient to racist cops due to their own racist views.
As you point out, there's no way to "test" for racism, and even if there were, firing people based solely on that would be uncomfortably close to punishing someone for "thought crimes". That's why people who try to tackle this problem don't try to eliminate prejudice, but to restructure and reform the system of policing so that the prejudices of individual officers or society at large don't have racist outcomes. There are lots ideas out there and many of them need to be tested in real world settings, but here's a overview of just a few:
Accountability: In any organization, it's inevitable that there will be some "bad apples" who abuse their power. The problem with the police, is that bad cops are almost always protected by prosecutors and their fellow officers and are rarely held accountable. Victims of police abuse can sue for civil damages, but since almost all (99.8%!) of this money is paid by the government (that is to say, the taxpayers), these settlements don't do anything to discourage these abuses. If police officers who abuse their power, whether in racially motivated ways or not, were held accountable then 1) there would be fewer bad cops and 2) people could better accept mistakes and lapses as just that.
Better training: One of the major aims of recent protests is the demilitarization of the police. In addition to the increasing use of automatic weapons and armored vehicles, which are disproportionately deployed in minority neighborhoods, many police departments have embraced a more aggressive and violent philosophy of policing that depicts police as "at war" with the public. This "Warrior Mentality" is best illustrated by Dave Grossman's Bulletproof courses on "Killology" as well as William Lewinski's Force Science Institute. As long as police see their job as "us-vs-them", then their conscious and unconscious biases will influence who they see as "the enemy" and lead to both general and racially biased abuses. If police could be trained to instead adopt a "protect and serve" mentality, the thinking goes, they might be better able to empathize and work with the public.
Off-loading police functions and decriminalization: If we accept that, in the real world, policing is going to continue to be applied in a racially biased manner, then by reducing, where possible, the extent of policing and the damage caused by minor encounters with the police, we can reduce the effect of this prejudice.
The quesion is based on a misconception: that police generally ARE racist. But a little research shows that black cops are perfectly willing to mistreat or even kill other black people, and that cops of any color are likewise willing to mistreat/kill poor white people. That suggests that the problem is in fact the general attitude of cops towards the civilian population.
The appearance of racism is mostly due to cultural factors, which might be the product of past racism in society. Black people, particularly poorer ones, are likely to live in mostly black urban neighborhoods where they are likely to frequently encounter cops. OTOH, many poorer* white people tend to live in more rural areas, and so can go years between encounters. Thus if the numbers of poor blacks and poor whites are equal, and there is the same probability that a given cop/civilian interaction will turn out badly, the resulting numbers will APPEAR to be caused by cops' racism, even though they're not.
*Like any bully, cops prefer to pick on those they perceive as weak.
