What sensor would you use to track how much liquid is left in a bottle?

The sensor would need to be part of the bottle.

Liquid left in bottle is bottle capacity minus removed content, but since you only want to know what is in bottle you don't need to know its capacity

Weigh bottle when empty We

Weigh bottle when part full Wp

Liquid in bottle Wl = (Wp - We)

So use a weigh scale as your primary measurement device

If you want volume you need to know liquid density

Or use a pencil to mark a line on the bottle and catch whoever is drinking your tipple

Set the water on this sensor, remove water to the level you wish to take action at and take a reading. Then write code to take action when that reading is reached.

https://www.sparkfun.com/products/11207?

Good luck!

Or this one to work in direct contact with the water itself.

http://www.ebay.com/itm/Water-level-sensor-liquid-level-Water-depth-detection-High-Sensitivity-Arduino-/251191540408?pt=LH_DefaultDomain_0&hash=item3a7c2ebab8

Arduino420: Or this one to work in direct contact with the water itself.

http://www.ebay.com/itm/Water-level-sensor-liquid-level-Water-depth-detection-High-Sensitivity-Arduino-/251191540408?pt=LH_DefaultDomain_0&hash=item3a7c2ebab8

That seems to have a pretty restrictive temperature range?

10.Operating Temperature: 10 ? -30 ?

Yeah he did not say what temp. Besides, until the whole rest of the planet gives up on that Metric crap, I would have not know what he meant anyway. ;-)