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:
- Add 100 mL of water to the bottle
- Blow into the bottle to hear the noise
- Match the sound your bottle makes with the online tone generator to determine the sound's frequency.
- Make a chart with frequency you found in #3 under the "x" and "100 mL" under a "y".
- Return to step #1 until the bottle is full!
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.