In book 20 of the Odyssey, Odysseus is still disguised as a beggar when he meets Eumaeus the swineherd, Melanthius the goatherd, and Philoetius the cattleherd. These three men do not recognize Odysseus and have no way to know who the beggar before them is. Therefore, they can and do speak freely before him without putting on any sort of show. They can express what they really believe, and they are free to reveal their true characters.
Eumaeus speaks kindly to the “beggar,” asking him if the suitors are treating him better or if they are still insolent. He clearly feels sorry for the ill-treated man before him. Melanthius, on the other hand, abuses the “beggar” and threatens him. Philoetius, however, remarks to Eumaeus that this “beggar” appears to be a great man and calls him a poor fellow. Then he speaks respectfully to the “beggar,” telling him of his disgust at the situation in his master's household and his strong belief that his master will one day return and put all those suitors out. When Odysseus does come back, Philoetius will help him in any way possible. Eumaeus says the same and wishes that his master would return home.
Now Odysseus knows who is loyal to him and who is not. Again, these men can speak freely to the “beggar,” for he appears to be no threat to them no matter what they say. In so doing, two of them reveal themselves to be Odysseus's allies. The dramatic irony, of course, is that these men are speaking directly to their master without knowing it (although the audience knows).