How can i tell if my snake is under weight?

[SlimeyLimey805]

ok, so when i had out my Dum today checking on how she was doing digesting her first meal and what not i noticed that she had some odd wrinkles near and around her head when she looks to the side or up to sharply and i am concerned that the previous owner either under fed her, or lied to me and never got it to feed which would explain how i got such a good price on her. and i am now concerned that she is not as happy and healthy as she could be..
BUT ANYWAY, i want to know from all of you better informed people than i, how can i tell if my snake is under weight or is in need of putting on some pounds (or ounces ahaha). i'll try to get my camera happy gf to come and take some pics asap. but if anyone has some pics of example, and some suggestions as to how to bring her back to a healthy size if that is what she needs.