How can you (the subject) know whether it's a placebo effect?
Because I felt it, and this may sound crazy, but...it changed me. The experience, I mean.
With the placebo, there would be no change in the subject, only the idea of change. Like taking a placebo for a headache, and then you 'feel' better, but you are convincing yourself of it. Eventually, your headache will come back, though.