Measure the length of the pig from snout to rump, and record it.
Download the length/age chart from http://www.ideas4ag-ed.com/uploads/3/7/0/4/3704787/fetal_pig_dissection_guide_day_1.pdf.
Consult the length/age chart to determine the appropriate number of days into gestation the pig was at the point of pregnancy termination.
Examine the pig's mouth. Determine how many teeth are present.
Identify whether the teeth are incisors, canines or pre-molars and where they're located along the upper or lower jawline.
Consult your veterinarian or the dental X-rays displayed at http://www.portec.com.au/thepig/disorders/head/anatomy/agingpigsteeth.htm to determine your pig's approximate age.
Measure the length of teeth in a pig whose teeth are all in. Determine the average incisor, canine and molar lengths. Consult your veterinarian to get a reasonable age estimate based on tooth size.