Media prevention campaigns aim to change the public’s attitudes or health behaviors. In addition to evaluations designed to measure the effectiveness of these campaigns in bringing about changes in attitudes or behaviors, it is common to conduct so-called “post-test” studies, which are intended to assess how the general public perceives the campaigns at the time they are aired and, in particular, to inform adjustments to communication strategies. The objective of this article is to present the strengths and limitations of post-tests, using as examples those conducted during the “Smoke-Free Month” campaigns of 2016 and 2017. These post-tests were conducted in November with samples of 2,017 and 2,039 individuals aged 15 and older, respectively, selected using the quota method and interviewed face-to-face. Awareness of the initiative is very high, as 80% of the population reported having heard of “Month Without Tobacco” or having seen its logo in 2017. When presented with the various media elements, 72% of respondents recognized at least one element in 2017 (compared to 62% for the first edition of Tobacco-Free Month in 2016, p<0.05). Furthermore, in 2017, the campaign was deemed to encourage smokers to reflect on their tobacco use by 52% of smokers, and to encourage them to quit smoking by 36% of them, with these proportions remaining stable compared to 2016 and ranking among the highest of anti-smoking campaigns conducted since 2004. Within a broader evaluation protocol that includes measures of effectiveness, post-tests provide valuable initial insights quickly and at relatively low cost to guide future communication strategies.