Hornworm pupation question

My hornworm looked like it was about ready to pupate (stopped eating, pulsating line on the back, etc.), so I moved it into a jar filled with coco fiber (with air holes) and put it in a dark place and I’m periodically misting the soil. If I understand correctly, it takes about 7-10 days for the pupa to fully form, at which point it can be removed from the substrate and kept on a moist paper towel. My question is: how can I tell when the pupa has actually fully formed? I don’t want to remove/disturb it too early and injure it, but how can I tell if it’s done if I can’t see it, since it’s covered in substrate?