I wouldn't really say there's a sure-fire, easy way to do it.. but it's to do with what key the music is in. You can tell what key a song is in by looking at the notes or the chords, or both. Once you know the key, you can usually figure out the melody not long after. It's just a matter of finding the starting and ending notes of the melody phrase and filling in the gaps. And since you know all the notes in a specific key, which I think are referred to as the diatonic notes, you will know which notes you can use to fill the gaps. You may not have to use all the notes.. and in some instances there will be a note that is not within the key but it is not that common in a primary or secondary melody. When that happens, it's just a matter of trying the in-between notes (chromatic notes) till you find the right one.