They sort of scrape round under the membranes and placenta with their hands and lift it all out and i would imagine for those who weren't in labour the plug comes out stuck to the outside of the lower portion of the membranes.

FWIW i had a spontaneous labour and a vaginal birth and no-one saw any plug apart from after a sweep 3 days before she was born i saw a small amount of snotty stuff - like the size of my thumbnail...

Bx