There are a few methods that rely on weight:
Using a balance scale: Empty two packs of juice to as close as half full as you can get, without going below. Check that they contain an identical amount of juice by confirming that they balance. Now put both half-full packs on one side of the balance, and on the other side, put one full and one empty pack. Both sides should have the exact same weight of 1 pack's worth of juice and 2 empty packs. If they don't weigh the same, empty some more out of the half-full packs and try again.
If you can pour juice into a pack, just empty one entirely, and pour juice from a full pack to an empty one until they balance.
Using an absolute scale: Weigh a full pack and an empty pack separately, and take the difference to determine the weight of the juice. Now just empty the full pack until it weighs as much as the empty pack plus half the weight of the juice.
And here's one based on volume:
If you only have a ruler: Freeze a full pack, unwrap it, and measure the edge length of the frozen juice tetrahedron. From this, calculate the volume. For any other pack that may or may not be half-full, freeze it (with the bottom face laying horizontally), unwrap it, and measure it its height. The volume of the truncated tetrahedron is straightforward to calculate, and the volume of a half-full pack will be exactly one-half the volume of the full pack.