Hello Scott,
What is the container type for this fluid? If it is glass or plastic, you may be able to use a capacitive proximity sensor.
It may be necessary to use a pair of sensors as a single proximity sensor will not be able to differentiate between nothing and a half-empty container.
On a related note, you may need to use some logic to make the determination. Let us know if you would like to explore alternatives.
Best Wishes,
APDahlen