You probably already use anti spam plugins on your site, but sometimes Spam gets through, so how do you tell the difference between Spam (bad comments) and Ham (good comments).
This post shows you the evaluation criteria I use before trashing or spamming a comment.
The Difference Between Trash and Spam
I have two routes for an unwanted comment, I either trash it or spam it, the differences are quiet important. Â If I find a comment that is real but I don’t want it to appear on my site (more about why I do this later) I will mark it as trash and bin it, if it is an actual spam comment I mark it as spam, the difference being, trash comments allow further comments, but spam comments potentially put that person or site on the Akismet blacklist.
I would ask you to use the Spam button aggressively to help combat this annoying and sometimes disastrous problem see When Too Many Comments Are Bad.
My Site, Â My Rules
Everything that is published on my site including comments reflects my site and my brand, if I allow low quality comments  to be approved what am I saying to the world, I don’t care what crap is published?  As a result I’m pretty picky what I pass through.
Also linking to poor quality sites, even through comments is thought to lower the value of your own site and could affect SEO rankings
So here are my criteria for evaluating suspect comments that have come through the spam filter.
It Needs To Add To The Conversation
I’m very strict on this one, so sorry if you have attempted to say great post in the comments, but I bin anyone leaving congratulatory remarks and anything that does ont add to the conversation of the post. Â You can disagree with what I have said, but your comments needs to add to the post.
Bizzare content
When you read the comment, it may look like it is written by a human, but it does not quite sit well with you, it’s too generic, whilst it could match your post, you are not sure.
When in doubt, bin is my motto
The site URL is Obviously Spammy
Look at the target URL or the site, is it obviously for  a spammy site, mark as spam.  Look for references to pharmaceutical, or any site selling stuff.  Add this to the next point and you have a spam candidate.
The Site URL Links To A Low Level Page Not Root
This always starts the red flag-a-flapping why would you link to a sub page if you were not trying to harvest links back to your piece of spammy crap. Â A legitimate comment links back to the root, you may want to link to a particular page in you comment to emphasise your point, but not in your site URL.
Irrelevant Links In Content
If there are any irrelevant links in your comment, you are obviously trying to harvest links back to your site for SEO purposes.  You gonna get a spam for that.  If anyone links in my comments, it better make sense to the post and comment, I often remove links but approve comments if they add value.
Foreign Languages
I’m not being xenophobic, but why comment on a site in language X in language Y? Â What are you trying to hide. Â I don’t approve any non-English comments on my site.
Further Reading
I’ve written a lot about spam in the past, check out this search result, “Dealing with Trolls” found me on a rant so lets mark the language NSFW.
Wrap Up
You wouldn’t let people write on the walls of your house, so why let people write on the e-walls of your site?
Obscure Reference Update: The title “Pass the man ham please” is from To Kill A Mockingbird by Harper Lee