According to Stevie B., A London based stylist, you can use your neck as a measuring tool to see if pants will fit your or not. Apparently, if the waist of the pants fit around your neck, then the pants will fit you.
I don't think this trick works for everyone, one user commented that it only works for certain sizes. Quote, "Only works up to a size 10... because as the stomach size increases the neck size doesn't."
Others swear by this hack.