I never heard of it either, here is something I found on the internet. I just copied and pasted it. I guess milk is ok in the pond? Mabye someone will tell us if milk in the pond is alright or not.
Here is a simple empirical method, which will let you know if your pond itself is leaking.
Milk jug check to see if a pond leaks
1. Get a straight-sided container (I used a milk jug with the sloping top cut off.).
2. Put the container in the pond so that its rim sticks out above the top of the pond.
3. Fill it with water to just about the level of the pond.
4. Turn OFF your fountains and waterfalls.
5. If your pond has a lot of shade, use two straight-sided containers, one in the shade and one in the sun.
Now you have two (or three) bodies of water, which are facing the same evaporation conditions.
6. Check the 2 (or 3) water levels daily for a few days. If the pond is leaking, the pond level will fall more than the jug(s) water level.
Milk test to locate where the leak is at
1. Let your pond lose water.
1a. When the loss rate changes, you are at the level of a leak. Record that level.
1b. When the loss stops (or matches the milk jog evaporation rate), you are at the level of the lowest leak.
2. Add about 1/4" of water to get the water level just above the leak.
3. Put some milk in a spray bottle.
4. Find where you think there is a leak, or systematically work your way around the pond edge.
5. Squirt a little milk in the water where you think the leak is.
6. If the milk simply dissipates, the leak is not there. If it is drawn through the side, you have a leak.
7. It is possible to have more than one leak at a given level, though that is not very likely. Visual inspection and perhaps a lot of squirting help you find all the leaks. If worst comes to worst, you have to repeat step1.
8. Remember to do this at each rate-of-loss-change point, as you may have leaks at each level.
