Placebo is controlled for in these kinds of experiments by using a comparison group exposed to a carefully designed placebo. To be very clear, you can test for

Placebo is controlled for in these kinds of experiments by using a comparison group exposed to a carefully designed placebo. To be very clear, you can test for the abscence of an effect. It is a little bit more challenging than finding a "true" effect, but it is feasible. It is a dangerous, and common, misunderstanding to not distinguish between "no significant effect" and "statistically significant zero effect".