When you say source do you mean in the email headers? Spam scores should always be in the email headers. There are some scripts [1] out there that try to reverse engineer all the spam scores but they are not perfect. Google has a tool specific to GMail. [2] To my knowledge very few mail providers hide this information but it can seem obfuscated as each provider may have their own proprietary in-house tools.

[1] - https://github.com/mgeeky/decode-spam-headers

[2] - https://support.google.com/mail/answer/29436?hl=en