There is an ecosystem of Web of Trust providers that is building solutions for this, Nostria will try to support multiple of them.
It's a fairly new development, but it's super important to fight spam and help users have a better experience on their social network.
Different providers weigh different metrics, depends on who follows who, who receives zaps, who you interact with, etc.
It's a personalized score from your perspective, you will always have perfect score.