Is Nabokov's Pale Fire really in iambic pentameter?

The poem is described as written in heroic couplets (iambic pentameter lines with end rhyme). When I read it, it seems to be to be free verse with end rhyme. I can't consistently identify the iambs (da-DUM, da-DUM, da-DUM) And when read aloud, it sounds like prose mostly with rhyme.

He's fairly consistent in having 10 syllables in each line. But they don't seem to be made up of 5 iambs.

Excellent question!

Well, let's figure it out.

Here are the first dozen lines of the poem, and let's mark the instances that don't seem to conform to the iambic canon:

I was the shadow of the waxwing slain
By the false azure in the windowpane;
I was the smudge of ashen fluff - and I
Lived on, flew on, in the reflected sky.
And from the inside, too, I'd duplicate
Myself, my lamp, an apple on a plate:
Uncurtaining the night, I'd let dark glass
Hang all the furniture above the grass,
And how delightful when a fall of snow
Covered my glimpse of lawn and reached up so
As to make chair and bed exactly stand
Upon that snow, out in that crystal land!
Retake the falling snow: each drifting flake
Shapeless and slow, unsteady and opaque,

As you see, in the first fourteen lines, or six heroic couplets, of the poem there are five deviations, which is in keeping with traditional English poetry patterns.

All of them are inverted iambs (trochees): no spondees, no dactyls, nothing radical: have you tried scanning Shakespeare?

Consider this sonnet, by John Keats, in which there are eight such deviations:

O Solitude! if I must with thee dwell,
Let it not be among the jumbled heap
Of murky buildings; climb with me the steep,—
Nature’s observatory—whence the dell,
Its flowery slopes, its river’s crystal swell,
May seem a span; let me thy vigils keep
’Mongst boughs pavillion’d, where the deer’s swift leap
Startles the wild bee from the fox-glove bell.
But though I’ll gladly trace these scenes with thee,
Yet the sweet converse of an innocent mind,
Whose words are images of thoughts refin’d,
Is my soul’s pleasure; and it sure must be
Almost the highest bliss of human-kind,
When to thy haunts two kindred spirits flee.

Et voila!