Half full or half empty?

There is a barrel with no lid and some wine in it.
- "This barrel of wine is more than half full," said Curly.
- "No it's not," says Mo. - "It's less than half full."

Without any measuring implements and without removing any wine from the barrel, how can they easily determine who is correct?

Tilt the barrel until the wine barely touches the lip of the barrel. If the bottom of the barrel is visible then it is less than half full. If the barrel bottom is still completly coverd by the wine, then it is more than half full.