You can't preview a song before you download it but you can preview during download. If you preview after ~20% download, you'll get some idea of the quality & content of the file. If the song sounds horrible, or isn't the one you want, you can kill the download

If you know some of the words of songs that you're looking for (but aren't too sure of the title)...try typing into Google words that you know are correct - lyrics and you might find out the song title (eg if you knew the words were blah blah blah, you'd search for blah blah blah - lyrics).