Ideally, the date and rightsholder will be noted in or on the materials (such as the copyright page in a book, or in the metadata of a digital file). There are also databases of records, many of which are accessible online for free.
Copyright is a federal law; in the US it is the responsibility of Congress, as stated in the Constitution. To ease international trade, most countries use laws based on the Berne Convention, which is...
If you have sheet music or liner notes, there should be an indication included. For a digital file, there should be information embedded in the metadata. If you don't have any materials like this,...