She will check for adherence to the publication's guidelines. She will verify the author of the piece. She will look for spelling and grammar mistakes. She may even hop on a search engine and see if your work has appeared in other poetry magazines. Preparation First, read the publication.
Back issues are usually available in an online archive.
Learn what kind of poetry the magazine tends to publish. Some newer online zines and websites will publish everything they receive, much of it drivel. Take into account your competition. Your poem will look much more prestigious if it is chosen to accompany other great works of writing. Some require a certain amount of words in a line, while others ask for poems under 20 lines.