How to find out if a preprint has been already published
(Added after @Wolpertinger's suggestion) Does the preprint itself include a link to the final paper? Arxiv abstract pages have a &quot;journal ref&quot; field where the author can include such a link, and some publishers mandate that a journal link must be added to preprints after publications.
Search on literature databases (Google Scholar, Scopus, etc.) for a paper with the same title.
Maybe the title has changed? Search for publications by the same authors to double-check. Has another paper with the same authors appeared in the years immediately following the preprint's date? Author changes are less frequent than title changes, in my experience, and most of the times authors are added, not removed.
If everything fails, try a full-web search for a sentence or two that appear in the paper.

If all these attempts fail, you may safely assume that it hasn't been published. If you want to be 120% sure, send an e-mail to one of the authors.
That said, note that a preprint still counts as &quot;previous literature&quot;, and you should cite it exactly as you would cite a published paper. However, be a little more wary about blindly trusting the results contained in it: there may be a reason this hasn't been published, in the end.