One of my favorite alcoholic beverages (Baileys Irish Cream) comes in bottles which are completely black. The bottle is so opaque you can't see the content even when holding it in front of a strong light source.
The problem with that is that this makes it really hard to tell how much is left in the bottle. It's also hard to estimate by weight because the bottle itself is quite heavy.
Is there a quick and easy way to estimate how much is left in the bottle without being able to look into it?
Blow across the top of the bottle like a jug in a jug band. If the noise is high, the bottle is full. If the noise is low, the bottle is empty. In between, the bottle is in between. Over time, when you listen to the various sounds your bottle makes, you can get a feel for how full it is.
Perhaps save a completely empty bottle to compare the sound that indicates you'll need to head to buy more Bailys.
If you were very serious about this, you can plot a frequency vs liquid graph. To do this, take an old empty bottle that is that same as the one you want to measure, and:
Make a graph with frequency on the x axis and the liquid volume on the y axis, and then post it on your refrigerator or near where you store your opaque bottle. Then, when you want to determine how full(empty) your Bailey's is, just blow in your bottle consult the graph.
If you keep the bottle in the refrigerator like my wife does, when you take it out
the condensation on the side of the bottle will tell you the level exactly within a few minutes.
In a rush? Breathe on the bottle.
With the bottle closed, give it a good shake. With a little practice, you can estimate how much is left by the sound and feel of the sloshing.
This method won't be as accurate as a weight measurement on a good scale, but it will be more accurate than gaging the weight by feel alone.
Obviously, this is not a good method to use with carbonated liquids.
Use a long straw or chopstick to help measure the contents of your opaque bottle. It will need to be longer than the bottle is tall.
Insert the stick into the mouth of the bottle. While a chopstick will allows you to more easily tell where the wetness in the bottle is, a straw will require you to remove it fast enough that the drops are still clinging to the side of the straw (I am assuming that the straw will be made of nylon or plastic).
A variation of this technique uses a weight on a string. The weight will need to be heavy enough to make a sound when it bottoms out so that you can get an accurate measurement. Use something that is safe, not lead sinkers used for fishing.
As noted by Ghost in the comments, you can form a seal around the top of the straw after it bottoms out. Remove the straw with the seal in place, bringing the liquid with it.
Put it into a big bowl of water. The emptier it is, the higher it will float.
When you first buy it, you could transfer it to a clear glass bottle. It won't be as attractive as the bottle it comes in, but it would definitely solve the issue!
(You could always print out a Bailey's label and attach it to your clear glass bottle, so it still has a good appearance.)
Take off the cap, use your phone torch to shine it in the top of the bottle, and turn your room lights off, you’ll see the remainder as a lovely pool of cream.
Tap the side of the bottle with your knuckle.
The sound changes slightly if you tap against liquid as against air.
So, tap repeatedly moving your knuckle from the bottom up, when the note changes, that is the liquid level.
Shake the bottle. Depending on the amount of liquid and its viscosity you will get totally different haptic and acoustic sensations.
A full bottle will have a great momentum and makes no sound. A half filled one will show a "delayed" momentum when the liquid splashes against the container and will produce intense sounds. A nearly empty one will make different sounds and won't show a noticable "delayed" momentum.
This allows for rough and fast estimations, but if properly practised I guess you can feel the content with an error of let's say 20%.
There is yet another method that is more commonly used for propane tanks and vessels with thinner walls. Essentially, you boil some water (2 cups or .5 L) and then pour it slowly down the side of the bottle. This will heat the bottle faster where it is empty, and much more slowly where your delicious Bailey's is still in contact with the bottle. So, if you run your hand down the side of the bottle, you should be able to feel where the temperature change happens.
Here is a YouTube video about how to do it with a propane tank:
With the cap off, tip it slowly while looking in the neck. The farther you have to tip in order to see the liquid, the less the bottle contains.
Microwaving it for thirty seconds to a minute (depending on an estimation of how much is in it) and taking it out will let you feel where it is warm (the liquid will warm faster) and how high the drink goes.
And this is sort of a no-brainer, but couldn't you just pour it into a cup, then back in the bottle to see how much it is?
The next time you get a new bottle: weigh it. Write down this weight somewhere. Then you can measure how much is left by weighing the bottle again, and subtracting from the full bottle weight.
A full bottle won't change its centre of gravity as its rotated as only a tiny gap of air remains. A partially empty bottle will start to shift its centre of gravity slowly as the air moves to one end.
A mostly empty bottle will shift its centre of gravity as soon as it begins to tip.
Visual aid:
