How do you know if you kill two birds with one stone?

Well, presumably you will know whether you threw one stone only, or more than one. And, if you succeed in killing two birds, you will likely find two dead birds, nearby. However, there is the possibility that you could mortally wound one (or both) of the birds and one, or both, could fly some distance from you, before cooperating, and dying. That would be hard to ascertain, as birds often hide very effectively before they die. So perhaps the best way would be to make sure that you are very accurate and powerful in your throw. From a pragmatic standpoint, it certainly helps if the two birds are standing in line, one behind the other. That way, you don't have to use geometry and calculus to figure out the ricochet angle of your stone's throw.