Who is Correct?

Last Problem:

What two numbers (X) come next in the following sequence?
1 6 2 7 3 8 X X

Answer:

4,9 – the pattern is +5 -4 or if you look at every other number, you can see two different patterns going on, 1,2,3,_ and 6,7,8,_ – so the next number of each sequence is a 4 and 9

Today’s Problem:

You are supposed to take 1 gallon of lemonade to the soccer game. You made fresh-squeezed lemonade in a 2 gallon jug which is perfectly cylindrical, but it looks like more than half full to you, and your soccer girl is certain there is less than half of the jug filled with lemonade. If you don’t use anything to measure with, how can you tell who is correct?