Consider this comment that appeared on a member blog:
Hey guys, sorry for being offtopic but what theme do you use? I’ve always loved the theme here.
Doesn’t sound out of line. A reader wants to know what theme you site uses. So here is the problem: It’s spam. This same message or variations of it it send to hundreds of thousands if not millions of blogs every day. Not that hard to do, considering that unlike junk mail, you don’t have to you don’t have to put a postage stamp on spam.
Here is another:
I have to admit, I really could not concur with you in 100%, but this is just my personal opinion, which indeed could be incorrect.
Again, not out of line. The writer seems to be giving you an opinion of your blog.
But it is completely fake. A robot might have read it, but the guy paying to have this spam sent out certainly never heard of your site.
So, how can you tell. Well, one certain giveaway is the link that the spammer is trying to put on your site. Hover above the username or handle on the comment. Look at the URL (Web address) that appears in the bottom of the browser window. If it points to a specific website, and that website’s name seems noncommercial, it’s probably OK.
But if the URL seems to be commercial or promotes a product (medication, handbags, ring tones, etc) it’s almost certainly spam. Some spammy links go to deeply internal pages and have longer URLs. But some legitimate commenters are choosing to deep link to their most recent post when the comment.
After you do this for a while, you will be able to look at a comment and tell it’s spam. In the two samples I cite above, you will note that the comment really isn’t specific to the content of your site or the post on which it is nominally commenting. It’s a general comment that can apply anywhere. Why comment to say they agree? And any serious computer user can can learn the theme you use by looking at the page code.
If you are not using a spam filter do so. There are several that can be activated on your Dashbord. Just click “Plugin” and chose “WP Spam Free” (my favorite) or “Akismet.”

Great advice – thanks!!!
BTW – NOT SPAM.