Can you see a mare in foal dilated? How can you tell?

My mare is pregnant and has been off and on showing signs of labor. One person who I talk to keeps asking me if she is dilated yet and gee I honestly don't know. Can you actually see that or are they just referring to the vulva bulging? I've asked her to explain but always get the same answer of "you know is she dilating?" Is there a way to tell?