Shaw uses several devices to communicate Eliza's frustration with her situation. Shaw primarily wants to reveal her annoyance at the insensitive Mr. Higgins, who treats her as less than human.

Shaw causes Eliza to expostulate or make noises of disagreement that express her frustration. Shaw's script repeatedly has Eliza saying, "Ah—ah—ah—ah—ow—ow—oooo!!!" Those sounds have no meaning except to convey  frustration in her situation.

Shaw underscores Eliza's Cockney background by having her respond physically to Higgins's rudeness. Shaw uses stage directions to indicate Eliza's frustration. For example, towards the end of the play, he directs that she start 

snatching up the slippers, and hurling them at him one after the other with all her force.

The slipper throwing uses the image of flying slippers striking Higgins to communicate, without words, Eliza's welling frustration and anger.

Shaw also uses dialogue. Eliza, when she can find the right words, is forceful in expressing her frustration. For example, after throwing the slippers at Higgins, she cries out:

There are your slippers. And there. Take your slippers; and may you never have a day's luck with them!

Shaw uses the exclamatory stage direction shown below to indicate Eliza's frustrated mood. The direction is exclamatory because it ends with two exclamation points, communicating Eliza's strong feelings. We read that she

gives a suffocated scream of fury, and instinctively darts her nails at his face!!

Shaw doesn't leave the audience or reader in the dark as to Eliza's emotions.