What does Menelaus reveal about Odysseus in The Odyssey?
I'm assuming you're talking about Book IV, when Telemachus visits King Menelaus and Queen Helen in Sparta. Telemachus has come there hoping for news of his father's return from Troy, or at least some details about Odysseus's departure from the sacked city. Menelaus and Helen do provide Telemachus with some information.
First Helen tells Telemachus of Odysseus's martial exploits:
He covered himself with wounds and bruises, dressed himself all in rags, and entered the enemy's city looking like a menial or a beggar, and quite different from what he did when he was among his own people. In this disguise he entered the city of Troy, and no one said anything to him. I alone recognized him and began to question him, but he was too cunning for me. When, however, I had washed and anointed him and had given him clothes, and after I had sworn a solemn oath not to betray him to the Trojans till he had got safely back to his own camp and to the ships, he told me all that the Achaeans meant to do. He killed many Trojans and got much information before he reached the Argive camp... (Book IV)
Here, Odysseus's prowess as a spy is explained. While there is a natural distaste for espionage (present in many soldiers and civilians of every age, including this one) on the part of Helen, she acknowledges that Odysseus's intelligence-gathering expedition was very helpful to the Argives (Greeks).
Menelaus then chimes in with the tale of Odysseus's work in the Trojan Horse:
All that you have been saying, my dear wife, is true. I have traveled much, and have had much to do with heroes, but I have never seen such another man as Odysseus. What endurance too, and what courage he displayed within the wooden horse, wherein all the bravest of the Argives were lying in wait to bring death and destruction upon the Trojans. At that moment you came up to us; some god who wished well to the Trojans must have set you on to it and you had Deiphobus with you. Three times did you go all round our hiding place and pat it; you called our chiefs each by his own name, and mimicked all our wives—Diomed, Odysseus, and I from our seats inside heard what a noise you made. Diomed and I could not make up our minds whether to spring out then and there, or to answer you from inside, but Odysseus held us all in check, so we sat quite still, all except Anticlus, who was beginning to answer you, when Odysseus clapped his two brawny hands over his mouth, and kept them there. It was this that saved us all, for he muzzled Anticlus till Athena took you away again.” (Book IV)
Again, Odysseus's cunning is presented as one of his central traits.
The next morning Telemachus asks Menelaus plainly what the fate of his father is, if the older man knows it. Menelaus then tells the tale of Proteus. After Menelaus and his companions had seized the god and forced hm to tell them their fate on their own homeward voyage, Menelaus had inquired if all the other Achaeans (Greeks) who had been left behind at Troy will get home safely. Proteus answers:
The third man,’ he answered, ‘is Odysseus who dwells in Ithaca. I can see him in an island sorrowing bitterly in the house of the nymph Calypso, who is keeping him prisoner, and he cannot reach his home for he has no ships nor sailors to take him over the sea. (Book IV)
So, when Menelaus left Egypt, at least, he knew that Odysseus was alive. This gives Telemachus some hope after all.