Suppose you might consider whether the de Broglie wavelength is a significant fraction of the likely size of the object in question: for classical objects, the de Broglie wavelength is totally insignificant compared with the size of an object; but if de Broglie wavelength is a significant fraction of the size of an object, you will definitely notice some quantum weirdness! Compare, for instance, the size of professor's de Broglie wavelength as compared with that of the He atom.