Well, you just knock on the barrel. lol. If you hear a shallow sound somewhere around the middle then it is half empty... Then how would you know if you are hitting at a height that is half of the barrel's hight?? ... maybe, You have to observe the shape of the barrel closely. If this is a regular barrel, then you will be able to see that the middle (or the height/2) is located at the point where the diameter is the greatest (just use your eyes)... it's something analogous to a sphere actually...

If this doesn't work, then you drink the wine, if you are half drunk then it was half empty if not then it was half full. LOL!