Q: how?
A: in my opinion, because "on 2 and 4" is such a famous phrase in our society, i've concluded that i think the tempo should be made to fit that whenever possible. So when there is any type of snare sound, clap sound, or strong mid-high percussion sound, occuring repeatedly on anything that could be percieved as beats 2 and 4 (out of 4), this should be assumed to be what's called the 'backbeat', and therefore on 2 and 4