Did Denethor see Frodo captured in the Palantir?

[Belegorn]

Originally Posted by FerniesApple

Denethor was a powerful and kingly man, but it was mostly pride imo.

Denethor was a High Man, a Dúnadan. He was a greater man than any other king unless it be a King of the Dúnedain.

Originally Posted by RotK, Minas Tirith

Denethor is of another sort, proud and subtle, a man of far greater lineage and power [than Théoden], though he is not called a king.

If you were to consider Denethor as compared to other men just recall the comparisons made of the Dúnedain to other men. The warriors of Rohan were like children next to them, and Éowyn saw that none of their warriors could compete with them. This would hold true of Denethor as well, being a Dúnadan.