If you can afford it, consider donating to the
NAACP Legal Defense Fund, especially if you can take advantage of an employee donation match. This is where we put most of our donation money.
It's hard to evaluate non-profit organizations, though
Charity Navigator can help with assessing whether or not an organization is scamming money or wasting it outright.
Measuring effect is more difficult. The NAACP LDF, though, produces litigation (that is often successful), and that is easy to see. Much of it is in the domain of voter rights and voter participation, which I think is one of the root problems in the United States, alongside Citizens United, poor education, technology-driven misinformation, and the Electoral College. (I'm talking about potentially addressable root problems here, not stuff like the people's fundamental inability to learn from observed mistakes or the possible inevitability of racism.)
Doctors Without Borders is another organization whose output is easy to check. They simply get medical care to a lot of people without skimming a lot off the top like the Red Cross does. And as it says in the name, they do a lot of work outside the US, though due to our incompetent handling of the coronavirus, they do a lot within our borders, too.