I find that everytime I've flagged something as spam (and it was deemed a valid flag), what helped me spot spam was:
- Has a link in the post
- The link in the post originates outside of SO
Additionally, about 1/3 of the cases I've found, the user who entered the post that I flagged, has a profile URL that points to the same domain as the potential spam post.
I would like to request a new tab in the review section, perhaps named "Has External URL".
I am only referring to posts that are already added to the review list; I'm not advocating that any and all posts with an external URL now be placed under review.