How does the author show the reader that both women know the identity of the murderer, her motive, and her method?

Glaspell describes the looks that pass between the women when they discover the dead bird that Minnie Wright has hidden.  They had already found the empty bird cage and discussed how much Minnie had changed since she got married.  Thus, they know that the bird is dead and that it was probably the one joy in Minnie's life.  When they find the dead bird, they immediately realize that they have found the motive--Mr. Wright killed Minnie Wright's only precious possession; so she killed him. The dead canary also provides a clue for the method of the murder (strangulation).  Mr. Wright wrung the bird's neck; so Minnie "wrung" her husband's neck with a rope.

Finally, at the end of the story, the women quietly decide to hide the evidence from the men.  Glaspell writes,

"Then Martha Hale's eyes pointed the way to the basket in which was hidden the thing that would make certain the conviction of the other woman--that woman who was not there and yet who had been there with them all through that hour."

This is the point when the women smuggle the bird out of the house (Mrs. Hale does so in her pocket).