If they post all the time with very low effort posts like, "Great post!" and "Very nice!" and they comment like this very frequently, they are comment spamming. Of course every once in a while it's ok, but if all of their comments are like this they are just putting out as many comments as possible as fast as possible to get money.
A comment should mostly only make sense on the post you are making it on. You could say "Great post!" on every post on the site, but "How do I know a comment spammer?" works perfectly for this post only, and maybe a few others, but you get the idea. :)