Are they adult pigs, or small enough to still be growing? If they are still growing, that could at least give you a relative ballpark based on weight and when they finally max out in the growth department (usually attaining full size within the first year, possibly a few months less.) If they are already adults, you could look for some clues... pigs tend to get thicker, curlier nails as they age, for instance. Young pigs have sharper, less twisty nails. Prime-of-life pigs (say from attaining full size to age four or four and a half) have a padded feel... older pigs feel bonier through the shoulders and hips as they lose muscle mass. Watching for those kinds of changes could give you some signs along the way, but it is a hard thing to gauge sometimes when they are adults!