NoBrandName said “How do you test placebos for their effectiveness? If placebos are your test group, what do you use in the control group, a different kind of placebo?”
The control group takes nothing at all. The whole point of testing a placebo is to determine whether it works better than nothing. You are testing the psychological component of the treatment. With normal drug trials, you are testing the drug itself, so you test it against a placebo to control for the psychological effect of treatment (Hawthorne effect).
Yes, I wonder how many of us are old enough to recognize it …