Rock-n-roll music is 4/4 time with a backbeat (emphasis on the off-beats, beats 2 and 4). While the same would also describe other styles such as Ska and Hip-hop, these other styles all have additional descriptors. The question "Is it rock-n-roll?" is equivalent to "Is it in 4/4 time with a backbeat?".
Rock-n-roll developed from "Race" Music (now called Rhythm&Blues) in the 1950s, and its typical tone and style reflects the "rebellious" nature of its birth. Often the same songs were recorded by black artists and white artists to be marketed to different demographics.