Summarize the following:
See what comes up. Someone else may have already thought of it before, or it may be too similar to something else. Once you've verified that your title is original, see what other results your search produces; this is a good time to find out if some part of your title might be obscure or overused, and what else people are searching for when they come across your fanfiction. Now that you've got a name for your fanfiction, go back over your story with this title as your new frame of reference. Does it do an adequate job summing up or characterizing your story? Does it make sense in context after reading it, or will readers fail to apprehend its meaning? Just as you proofread your writing to look for inconsistencies, double-check your title to make sure that it's a fit for the completed work. Rereading your fanfiction will give you another opportunity to mine for other potential titles that might better serve the story. Ask a friend to look over your fanfiction and give you their thoughts, particularly on the title choice. You wrote it to be read, so this will also give you your first opportunity to have other eyes on your story and receive some initial feedback. Have your friend point out the strengths and weaknesses of your story where the relation to the title is concerned. Ask the person reading your fanfiction to tell you what they think of the title before and after reading the story. If it's not strong enough to interest them before they read the story, you may need to rework it. Find a friend that is impartial or more likely to give you honest criticism. Ask them to be specific: hearing "it's good" benefits no one. Before you commit to your title, consider what inspired your fanfiction. If it directly borrows from source material, make sure the title you've chosen is in keeping with the style of the source while also being original enough to distinguish itself as your own original work.
Run a search for your title. Reread your story. Have someone else read your fanfiction. Stand out from your source.