The detail that allows the reader to predict that Alice has stepped into an unusual place is: "...and the very clock on the chimney-piece (you know you can only see the back of it in the Looking-glass) had got the face of a little old man, and grinned at her."
This detail indicates that the objects in the Looking-glass room are not what they appear to be in the real world and introduces the element of whimsy and absurdity that characterizes the unusual world Alice has entered.