From what I understand, there are a couple ways the illusion is performed, depending upon the magician's technique, that would mean different answers to your question.
1: No, they are fake. Some magicians use a Special-Effects animatronic set-up with rubber feet and a simple motor to produce some basic flexing or rocking motions by remote control. The Lovely Assistant kicks the fake feet into position as she enters the box, then tucks her own legs up against her chest so that she's scrunched in a fetal position inside the top half of the box when the sawing occurs. When the box is rejoined, the Lovely Assistant hides the false feet back in the lower half of the box before exiting.
2: Yes, they are real. This version of the illusion is performed much the same as above, only the magician has a second Lovely Assistant already hiding in the bottom half of the box in the fetal position before the first Lovely Assistant enters the top half. LA#2 sticks her feet out of the box as LA#1 enters, and provides a more realistic wiggling than the artificial version.
If the box is presented on a trolley or cart covered with a curtain, or other such means to conceal the presence of a second girl hiding below the box, you're probably dealing with real live feet. However, this is not always the case, as many magicians hire their Lovely Assistants for their talents as contortionists, so that they can squeeze into the box completely and the magician can make a point of showing that there's nothing hidden below.
The only way to tell, I suppose, is to check the range of motion from the feet. If they seem to move in a predictable and repetitive pattern, with little flexibility, odds are you're dealing with artificial specimens.
