How will you know if a song is copyrighted?

Anything written since 1978 will be protected for the life of the author plus 50 years in most countries; the US and some others have extended this to 70 years. Most songs written 1923-1977 are protected for 95 years from publication; some 1923-1963 songs may be in the public domain if they were not properly renewed, but this is difficult to prove. The safest option is to assume anything 1923-present is protected.

Recordings are even more confusing. Most older ones will not enter the public domain until 2067, but post-1978 recordings may enter the public domain as soon as 2049, because their copyright term is for the life of the author plus 50 (or 70) years.