There are several clues:
- The poster has less than 5 posts, typically only 1 post.
- The post is usually nonsensical
- There is a link in the post and if there isn't, the poster comes back in 24 hours to add one
The link is usually hard to spot. In the post above, it is a blank underline.
The real question is, if the spammer is paid by the click, why do they typically make it so hard to find the link? The only speculation I have is that they are paid just for the posting because the software the payer is using only finds the link and doesn't inspect it to know that it is nearly invisible.