Hang it on a scale. I have seen people use a fabric tape to measure girth to estimate weight but no way that is accurate.
I made it point some years ago to tape and then physically weigh a couple of dozen hogs (Boars & Sows) and found the method to be pretty darn accurate.
I'm sure in some locales it would vary some. Habitat and the condition of the hogs would make a difference. But...lacking an actual scale...I would wager the tape method comes closer to estimating the true weight of the animal than does simple 'guessing'. Unless you have killed and weighed many hundreds of hogs...it is easy to misjudge their weight.
Also, I have found that hog hunters (not all) are bested only by fishermen in their willingness to 'embellish' the weight of their quarry.