Barrel Puzzle?

Question

'A' says that a barrel of beer (without a lid) is more than half full. 'B' says that it is less than half full. Without using any measuring instruments, or other vessels to transfer the contents, how can they tell who is right? Both of them are looking into the barrel, but they are not sure as to who is right. So what do they do?

Answer 1

They draw a mark to indicate the level of the liquid inside the barrel.
Then they turn the barrel upside down, and if the liquid is still at the same level as the mark, the barrel is half full.

*add - I've just realised I'm completely wrong.
Ignore the above.
The answer is I don't know.

Answer 2

Tilt the barrel slowly and gently on its base. The beer level closer to the ground would slowly rise up. Continue the gentle and slow tilting until the beer level just touches the rim of the open top, without spilling over.
At this point, observe the level of of the beer inside the barrel at the opposite, bottom end. If you can see the bottom part exposed even a little bit, it would mean that the beer level is less than half full. If the beer covers the bottom completely and its level is above the bottom, then the beer is more than half full. If the level at the bottom just touches the bottom rim, then the beer is exactly half the barrel.