This is especially true for the new writer. How to avoid scammers is the question, to which there is no pat answer.
The main thing to remember is to do some checking at places like the Writer Beware page of the Science Fiction and Fantasy Writers of America, Inc. (http://www.sfwa.org/beware/),Preditors and Editors (http://anotherealm.com/prededitors/), and Writers Weekly (http://www.writersweekly.com).
You may also ask in writing groups you belong to, or anyone you know who is a writer about their experiences. If you do not belong to a writing group, consider joining one.
Some unscrupulous places out there will offer to publish your poem while asking you to purchase an anthology or some such. They will usually ask you if a poem is yours or list a contest. Many writers have been burn by such tactics. All I can say is, is careful.
Even if you see contest listed in writing magazines, still be careful and read carefully before you enter your writing. A small reading fee or entry fee is one thing, but if there were a reading, entry, or some other fee or a request to purchase anything, I would run not walk away from that contest.
If you wish to entry a contest, MAKE SURE you read and follow the guidelines for the contest and follow them to the letter. The same goes for any piece you wish to submit for publication; FOLLOW THE GUIDELINES!