Or maybe you’ve constructed a square-circle detector; no experiment would let you tell the difference, no?
Take the thing apart and test its components in isolation. If in isolation they test for squares and circles, their aggregate is a square-circle detector (which never fires). If in isolation they test for pink unicorns and invisible unicorns, their aggregate is an invisible-pink-unicorn detector (which never fires).
Take the thing apart and test its components in isolation. If in isolation they test for squares and circles, their aggregate is a square-circle detector (which never fires). If in isolation they test for pink unicorns and invisible unicorns, their aggregate is an invisible-pink-unicorn detector (which never fires).