A pressure (vacuum) sensor could do this. You could measure the vacuum and estimate the height of the liquid in the straw from it if you wanted to do a fancy animation of some sort. Difficulty: keeping the sensor port clean.

A barometric pressure sensor could do this. Baro sensors measure absolute pressure, so you could measure the vacuum and thus estimate the height of the liquid in the straw from it if you wanted to do a fancy animation of some sort.

Related: Low cost barometric pressure sensor

Difficulty: keeping the sensor port clean.