It goes on and on!
What catches my attention is if there's a response to a very old post, or if the spammer has copy/pasted from another website (Reddit is popular for some reason!), or the post just appears to be absolutely meaningless.
Another way that the spammers try to get around any suspicion is to post a seemingly legitimate post. Anyone on the forum might look at it, can't see any problem, and move on. Then the spammer comes back a few days later and edits their post to add their spam or spam links. Here's an example thread:
http://www.emaildiscussions.com/showthread.php?t=76929
If I'm suspicious I also check out their profile, and if the spammer has what appears to be a spam link as their home page, I consider that this is a spammer. In the past I have reported this to one of the mods, but he (there, I've narrowed it down a bit!) disagreed and so the home page link remained.
We all want a spam free forum, and we don't want to be conned into responding to a post for someone whose only aim is to spam us.
Please everyone, when reading posts, always think if there's a legitimate reason for the post.
If not, please report the post to the mods.