How can one tell if the copyright law has been broken?

If a work has been copied, altered, distributed, or performed/displayed without permission from the copyright holder or an exemption in the law, that copyright holder's rights have been infringed upon.

Deciding whether a work is infringing is not always straightforward, just as it's not always easy to tell a counterfeit from the real thing. In cases where musicians have been taken to court for copying another's song, both sides will call in expert witnesses with specific examples and comparisons.