If you see them suddenly stop what they are doing, ears both facing forward (perked up)...then you know they have 'alerted' on something. They may or may not run. But IF they throw their nose up in the air, SHOOT...'cause they are getting ready to vacate the area.
Often times when a hog thinks it has detected a threat (or something suspicious) via its sense of Sight or Hearing...it will want to confirm/validate that notion using one the other senses (Hears something...wants to see it too).
But NOT with the sense of smell. They smell something they don't like they are GONE!
Spartans ask not...how many, but where!