How do you decide whether the email is a spam?

I was assigned to monitor the email flow of my company, the default policies to filter spam will label a spam header to the email if particular has spam content. However, how do you know it is a false alarm or not? Currently, to check the email source, I will trace the domain host and IP with MXtoolbox( you can search google for it). However, sometimes this MXtoolbox will detect the domain host as spam but not the corresponding IP. The IP could not be found but the domain host is exist. I want to know is there any better way to determine whether an email is spam. Thanks