It's hard to say. The only way to really test whether your song's plaguarized is to go to court with it . A song is copyrighted in essence as soon as it's been put to paper but you have to prove that you were the one who originally wrote it. I think generally, most songwriters stick to the honor system cause the last thing you want to be known as is a ripoff artists but on occasion there's been songs that have gone to court and lost out because they were too similar to the originals. I can't remember the songs names but one was a George Harrison tune called My Sweet Lord which was almost a direct ripoff of another song. John Lennon also used to take the music to other peoples songs and write new ones too them. And Tears for Fears almost copied a Beatles tune note for note. Whether or not they're considered copyright violations though, can only be proven in court. And most people don't bother because of the costs involved.