To get to know what a community is like, spend some time there and read and get a feel for the kinds of articles that are relevant to that community. Both “Relevant” and “Spam” will be good examples of what to post and what not to post.
When in doubt, read don’t post. Posting will NOT earn you REL Coins and will only irritate active members if the post is irrelevant. As a rule of thumb, if you don’t know what will or will not be relevant, you should not be posting in that community. You can still bet/stake on posts.