Toward the end, the shape of opponents' hands can be read by "counting" (playing out two or three suits and inferring the shape of the remaining one(s)).
In the beginning, one looks for more subtle clues. If a lead is fourth best, one might look in hand or dummy for lower cards to see if the leader might have a fifth or sixth. If opener has bid three suits they are likely to have a singleton or void in the fourth. If one defender has bid they probably have the bulk of the defenders' points; if neither has bid, the points are (probably) evenly split.
What are other ways to draw inferences about opponents' hands after hearing the bidding, and seeing one or two rounds of play?