She will taste as like this as a crab does to a crab. Thou canst tell why one’s nose stands i' th' middle on ’s face?
Lear is negotiating his accommodations with his daughters and it is not going well. The Fool taunts Lear by saying that Regan will be kind where Goneril was not:
He then reverses his message and says what Regan will be like as opposed to what she will not be like:
The following riddle is a lead in to his next jibe at Lear's senseless choices:
Thou canst tell why one's nose stands i'
the middle on's face?
[...]Why ... that
what a man cannot smell out, he may spy into.
In other words, Lear is being jeered for not knowing the truth about his daughters: he should see the truth or at least smell out foul deeds in the wind.
I have to agree that the fool is warning Lear. He is making a point of showing Lear that some things are exactly as they seem (instead of how they appear). Many of Shakespeare's plays were full of conflicts between appearance and reality. The fool is trying to point out how he feels about Lear's daughters and Lear himself.
This is an excellent example of the way that the Fool uses his status as a fool, as one who speaks nonsense, to actually speak truth in the guise of nonsense to his master, Lear. The Fool is actually, ironically, one of the wisest characters in the play, as he sees through the role of devoted daughters that Regan and Goneril play and identifies them for the treacherous individuals that they are. This speech therefore reflects this reality as the Fool tries to warn Lear.
The comments above about crabs and apples are correct, so I'll try to have a go at the rest of the phrasing you asked about. Interestingly, none of the three major editions of the play -- Arden, Oxford, and Cambridge -- says much of anything about the comment on the nose. Here's the phrasing:
- Fool. She'll taste as like this as a crab does to a crab. Thou
canst tell why one's nose stands i' th' middle on's face?
- Fool. Why, to keep one's eyes of either side's nose, that what a
man cannot smell out, 'a may spy into.
The fool seems to be implying that something figuratively "smells" about the conduct of Goneril and Regan, but that if Lear cannot "smell out" their treachery, his eyes at least give him the chance to "spy into" it. Lear, however, is distracted by guilty thoughts about Cordelia ("I did her wrong") and doesn't reply to the fool.
In this scene, the fool is telling Lear that he is making a mistake trusting Regan and Goneril. The part about crabs refers to crab apples and it means that they will both taste the same, like crabapples. He is saying that they are both sour people. The part about the nose is not understandable unless you read the rest of the scene. The fool says a bunch of silly things that lead to the idea that a snail is smarter than Lear because it carries its house on its back and can't give it away.