Opal glass will scatter the light, it will not work.
Clear or smoked glass would work but you will have reflections from the glass back to the phototransistor.
Maybe a slight mod of this idea where there are two sets of emitter/detectors under a transparent layer. When there is no glass present, the ratio of light picked up by the two detectors will be different that the ratio when a glass is present.
I wonder if one could use a piezo to sense the difference in resonance frequency of the surface with and without a glas on it ...
Me again .. you can buy spirit optics for around $4 that fit a bottle
The liquor suppliers just give them away to bars. I was at a bar in Newbury and the barman gave me a couple just because I was wondering about them. They may be a little too manual for this topic though.
Yes, but I do not want to have a physical switch that is pressed by the glass. There are also lightweight plastic shot glasses that are not heavy.
I know, I run the pump in reverse after idling.
Okay... I will try that
This could work. If my version with opal glass does not work, I will try your solution. Thank you very much!
That would be too easy 
Thank you all for your suggestions. I will try on.
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