Normally, if a swallowtail pupa is in diapause, if you hold that pupa up to a light source, the region around where the eyes form is opaque (as a matter of fact, the whole pupa is opaque) where light cannot pass through the pupa.  However, when the pupa is developing and the eyes are forming up, if you move that pupa near a light source, you will see where a portion of the light will pass through the chrysalis AND, at the same time, you will be able to see the darkening of the eyes adjacent to that region as shown in this photo.  As the butterfly develops further, you can see the wings developing; but this can be much more subtle than the obvious sign that a monarch imago is developing inside its chrysalis.