You could try "faking" an industrial load cell. My idea is something like this. The glasses are put on a small plate which is held by a vertical spring(s). The weight of the empty glasses lines up an electrical connection. Further weight (i.e. full glasses) pushes the connection past the point of connection so that there is a weight range of which the filler will operate.
I guess one problem of my setup would be the actual connection of course. My first guess would be magnetic, but they would have the effect of holding the plate from falling properly. You would need some sort of lubricant and a separation plate.
Oo, another idea (curious cause I might be a boozehound myself). The plate is again on a spring (but just a soft spring to keep it from falling quickly). The plate is attached to a vertical gear rack. That rack is then in turn connected to a pinion, which is attached to the bottle. Now, the fully mechanical design works like this.
The glasses are placed on the plate. The weight of the glasses pushes the rack down, which turns the pinion. The bottle is then tipped over enough to start pouring. The liquid is poured into a manifold (tube) which has holes cut into it, which will be lined up with the glasses.
Upon filling, the glasses get heavier, and the plate pushes farther down on the spring. When the glasses are full, the racked will be pushed down far enough such that the rack becomes disengaged from the pinion. With nothing holding the pinion, it rotates back to its starting point (by way of another spring).
The problem here would be implementing some sort of ratcheting action on the rack to enable them to become engaged again without disturbing the pinion.
