Roger Ebert's assessment of the rings left by Claudette's gin Rickey was completely off base, incorrect in both facts as presented as well as needlessly snarky.
Bubba Rock mentions the gin Rickey fixation that Claudette has in their first scene. Bubba also insists Dave use a coaster in this scene, in addition to getting in a spat with Claudette, as Bubba is annoyed by her not using one.
Later, Claudette is seen on Dave's porch drinking from her stainless steel thermos. You can clearly see that she uses a very small cup that also serves as a lid for the thermos, not much bigger than a shot glass, with a wider base. There are only a couple of rings, not many, as if to suggest she was not setting the cup down between sips.
The rings on the table in the porch scene, and later near the end, are residue from the gin Rickey from when Claudette pours the drink from her thermos. Gin Rickeys contain lime, a distinctive taste and smell that will leave residue after evaporating. Dave also notices the rings on the porch table.
In the late reveal, when Dave spots the rings after killing Romero, Claudette's pitcher is in the same shot, tipped over next to the rings of lime and gin, which again, is from pouring into a small cup.
During that scene near the end, Dave, a former homicide detective and recovering alcoholic, Dave tastes the residue. The taste of gin might just be known to a former big time drinker. Lime is again, hard to miss. Dave can also see the thermos, again, tipped over next to the drink rings.
Mere seconds later, in the very next scene after Dave discovers the leftover gin Rickey thermos and evaporated residue, he confronts Claudette at the Rock residence, who is drunk, this time drinking from a glass. Dave asks her, "Where's your little thermos?".
Thus, Ebert's observation that the residue must have come from condescension, was incorrect. Dave saw the rings which were a result of pouring into a shallow cap from a thermos, the thermos on Claudette and in the cleaners later, knew the drink she drank, and tasted the distinctive residue, which is how he reasoned that Claudette was behind the hit. There is no factual error concerning this part of the plot, whatsoever. Roger Ebert was wrong.
