Let's say I come across a man lying in the street. He's bleeding from his shoulder, and he's unconscious. There's a smoking gun nearby, and the hold in his shoulder is about the size you would expect from a bullet wound. You can see a spot on the wall behind him that looks like it was hit by something small and fast. Adding on to all of this, you heard a gunshot, which is why you came here in the first place.
Now, you didn't see this guy get shot, and you obviously can't reproduce the incident as it happened, but when you call 911 and tell them that this man was shot, are you guessing?