I have noticed that a number of people have modelled glasses with liquids in them, but to me even with hdri the render looks totally unconvincing. I think the model has not accounted for the interaction of the liquid with the surface. Liquids wet surfaces. A liquid drop on a surface will have a definite contact angle with a surface. For some surfaces and liquids the contact angle is small so that liquid spread out over the surface. In the case of liquids in a container interaction between the container’s surface and the liquid leads to the formation of a meniscus.
The first picture below, for water in a glass, illustrates that there are subtle reflections around the edge of the liquid due to the meniscus. The next shows that the meniscus has quite a width, its the band below the red band in the center of the glass. Finally, reflections on the surface of the water are quite hard to obtain, and it appears that only the most intense light sources (a light bulb) are reflected. The ring clearly visible near the inner wall of the glass is probably due to the curvature of the liquid surface and shows that the effect the meniscus has on specularity extends quite a distant from the inner surface of the glass, ie there is still a detectable curvature, albeit small, in the surface some cm away from the inner wall.
If anyone would like to model a glass of water with a meniscus, please post your results here. I’m going to give it ago, but I’m a noob :-? .