Yeah the best way to find out who did it is to catch them doing it. If it was only one baby I wouldn't worry about it too much, but if they start doing it to the rest of the litter then I would cull. Also check the watering system for your ASFs. The only time mine have cannibalized an older (more than 1-2 days old) baby, it was because the water bottle wasn't functioning right and they couldn't get any water out.