With the millions upon millions of songs in existence, how can a songwriter be certain they have not plagarize?

How can part of a tune or words be ascertained as original considering the mive numbers of songs? With more and more corporations or celebrities who own phrases or words where does freedom of speech begin and end? For example Rosie Mcdonnel "owns" Rosy, Donald Trump "owns" You're Fired and the Intl Olympic Committee "owns" Olympic and thereabouts.