There are many ways of assessing this, but the simplest is to just ask participants to self-report their drug use, and then drug test them [1][2][3]. There are also other ways of assessing real rates of drug use, such as looking at drug-related emergency room visits and stuff like that, but I don't think we need to go into that.How would you know that they lied more about drug use? It is a fine critique of the self-report statistic. But how would you tell which cohort is lying more than the other?