If we speak last of the “folly” of Manwë and the weakness and unwariness of the Valar, let us beware how we judge. In the histories, indeed, we may be amazed and grieved to read how (seemingly) Melkor deceived and cozened others, and how even Manwë appears at times almost a simpleton compared with him: as if a kind but unwise father were treating a wayward child who would assuredly in time perceive the error of his ways. Whereas we, looking on and knowing the outcome, see now that Melkor knew well the error of his ways, but was fixed in them by hate and pride beyond return. He could read the mind of Manwë, for the door was open; but his own mind was false and even if the door seemed open, there were doors of iron within closed for ever.
For Manwë was free from evil and could not comprehend it, and he knew that in the beginning, in the thought of Ilúvatar, Melkor had been even as he; and he saw not to the depths of Melkor’s heart, and did not perceive that all love had departed from him for ever. （The Silmarillion）
No one, not even one of the Valar, can read the mind of other 'equal beings':* that is one cannot 'see' them or comprehend them fully and directly by simple inspection.
*All rational minds/spirits deriving direct from Eru are 'equal' - inorder and status - though not necessarily 'coeval' or of like original power".
But to force such a revelation, or to induce it by any lying or deception, even for supposedly 'good' purposes (including the 'good' of the person so persuaded or dominated), is absolutely forbidden. To do so is a crime, and the 'good' in the purposes of those who commit this crime swiftly becomes corrupted.
（Notes on Motives in the Silmarillion，HoME 10）
How otherwise would you have it? Should Manwë and the Valar meet secrecy with subterfuge, treachery with falsehood, lies with more lies? If Melkor would usurp their rights, should they deny his? Can hate overcome hate? Nay, Manwë was wiser; or being ever open to Eru he did His will, which is more than wisdom. He was ever open because he had nothing to conceal, no thought that it was harmful for any to know, if they could comprehend it. Indeed Melkor knew his will without questioning it; and he knew that Manwë was bound by the commands and injunctions of Eru, and would do this or abstain from that in accordance with them, always, even knowing that Melkor would break them as it suited his purpose. Thus the merciless will ever count on mercy, and the liars make use of truth; for if mercy and truth are withheld from the cruel and the lying, they have ceased to be honoured.