In King Lear, when does Lear recognize that his two older daughters are evil and that Cordelia was the one who really loved him?
Lear's enlightenment has more to do with himself than with his two daughters. He does not necessarily realize that they are evil. He realizes that they are liars and that they care nothing about him at all. An especially revealing soliloquy comes in Act IV Scene VI, where he says:
They flattered me like a dog; and told me I had white hairs in my beard ere the black ones were there. To say ‘ay’ and ‘no’ to every thing that I said!—‘Ay’ and ‘no’ too was no good divinity. When the rain came to wet me once, and the wind to make me chatter; when the thunder would not peace at my bidding; there I found 'em, there I smelt 'em out. Go to, they are not men o' their words: they told me I was every thing; 'tis a lie, I am not ague-proof.
Lear was born a prince and then became a king. This gave him an inflated opinion of himself, as is the case with many people who are born to privilege and high social status. He is like many selfish aristocrats, but even more so because he is at the very top of the social ladder. He thinks he possesses great value in himself and doesn't realize his value is in his property and power--until he gives them away! He might be compared to the young Prince Edward in Mark Twain's novel The Prince and the Pauper, or to Harvey Cheyne in Rudyard Kipling's novel Captains Courageous. Lear thought he was such a wonderful man that everybody must love him, although they were all pretending to love and admire him because he was the king. He thought he could trust his two daughters to take good care of him and put up with all the antics of his second childhood. Goneril and Regan did not really conspire to hurt their father; they just didn't care about him. He was nothing but a nuisance. They were glad to get rid of him.
Shakespeare may not have intended to portray Goneril and Regan as evil but only as fairly typical human beings carried away by greed and power. The Fool understands what people are like. He was probably a retarded pauper, the village idiot, despised and abused by everybody until Lear picked him up in some village where he found him living among the hogs and chickens and made a court jester of him. Throughout the play Lear is learning what the Fool has known all his life. It is a cold, cruel world.
The original question had to be edited down. I think that Lear realizes the true nature of his daughters too late. This is what creates the tragic element to his characterization. In the drama, I think that Act II, scene 4, is where Lear's recognition becomes evident. Having been rejected and cast out by one child, the father experiences it with the other one. It becomes clear that the children's declarations of love were hollow. Lear recognizes this. The statement, "I gave you all" becomes significant in this recognition. Lear understands that what he gave to his children is not reciprocated. The label of "unnatural hags" also helps to enhance the recognition that his two children that he supported do not support him. The fool's metaphor of the rich parent who gives their child everything only to see betrayal result is another level of recognition that demonstrates the harsh truth to Lear. The fool also reveals the flip side to this coin. He talks about how poor " fathers that bear bags Shall see their children kind." There is a convergence of reality upon Lear at this point. It is this acknowledgement in Act II, scene 4 where the emotional reality of Lear's world settles on him. It is in Act II, scene 4 where Lear understands the truth about his emotional relationship with his children. It is also at this point where the storm is about to enter, a moment where Lear's own sense of self will be transformed. Such an understanding was brought on by the rejection he experienced at the hands of daughters who said they loved their father, a love that only existed in words.
It is hard not to feel some measure of sympathy for Lear in Act II scene 2, which is when Lear comes to realise the consequences of his actions as both his daughters reject him and basically leave him to go out in the cold by himself. He thought that devolving all responsibility on his daughters would leave him free to live a life without responsibility and the duties that were so onerous to him, but he is very quickly made to see that his dream of how he wanted to spend his last years is not to be. When Goneril and Regan confront him with their ingratitude by showing him he is not welcome in either of their households, Lear shows his recognition of their evil with the one line speech, "I gave you all." The fact that this is all he says in this line indicates the kind of disbelief and horror that he experiences as he is forced to confront the fact that he has given all of his power to his two ungrateful daughters who have abused that responsibility. Note what he goes on to say as he addresses the gods:
You see me here, you gods, a poor old man,
As full of grief as age, wretched in both.
This comment on Lear's mental state proves that it is at this point in the play that he realises what he has done, and that Regan and Goneril are actually evil and he has banished the one daughter who loved him purely and truly. The audience see Lear stripped of his self-confidence and arrogance, an old man, deserted and rejected by those who should love him and care for him.