This famous and sometimes misunderstood line from "Ode on a Grecian Urn," by Keats, is surrounded by lines of iambic pentameter. This line is a variation, which is natural since it's the centerpiece of the stanza and, perhaps, the poem.
If you check the dictionary to see where the stress falls in the word beauty, you'll find that it lands on the first syllable. That takes care of half the line. And, since beauty is the first word of the line, you automatically know then that this will not be an exact line of iambic pentameter. Thus, I would scan the line like this, with stressed syllables in bold:
"Beauty is truth truth beau ty."--that is all
So you have the following:
Trochee iamb spondee iamb iamb
Again, it is a variation of iambic rhythm. Hope this helps.