The difference between blank verse and prose lies in the rhythm, but it can certainly be more difficult to identify the difference between the two in the works of early modern writers other than Shakespeare—simply because, if we are used to the works of Shakespeare, where the distinction is very clear, the writing of a playwright like Webster will seem less rhythmic. Shakespeare's blank verse tends to use very regular iambic pentameter—each line spoken will have five clear points of emphasis, each of which is known as a foot, or iamb.

Webster's language here is less distinctive. However, he adheres to the same general set of rules for versification as other playwrights of the time: namely, iambic pentameter is used to indicate a character of higher rank, whereas more common characters will speak in prose. Occasionally, rhymed verse will be used, often to end a scene.

Webster marks out the first full line of the Duchess's speech in clear iambic pentameter to make it obvious that she is, in fact, speaking in blank verse rather than in prose:

The MISery of THOSE who ARE BORN GREAT

One can shift the emphases slightly—we might place emphasis on the "Y" of "misery", for example, rather than on "born"—but it is fairly difficult to pronounce this line without sounding out the five clear iambs which mark out blank verse during this period.

The wooing scene of John Webster’s play The Duchess of Malfi (Act 1, scene 3) is written in blank verse, as can be seen by consulting, for instance, the edition of the play in the Norton Anthology of English Literature. However, the rhythms of the verse are supple and often unpredictable, often giving the speeches the flavor of prose.  Few of the lines in this scene, for example, are written in the kind of regular iambic pentameter made famous in Christopher Marlowe’s line (from Doctor Faustus) “Is this the face that launched a thousand ships?” Occasionally Webster offers such a line, as in the Duchess’s comment, “And as a tyrant doubles with his words” (1.3.146). Even this line, however, might be scanned as slightly irregular, especially if “as” and “with” were not heavily stressed in reading.  More typical of the speeches in this scene, however, are lines such as these two by the Duchess:

The misery of us that are borngreat!
We are forced to woo, because none dare woo us . . . (1.3.144-45)

The second line here is particularly difficult to scan, especially in its second half. Webster, like Shakespeare in his late plays, seems to have been writing verse that was often hard to distinguish from prose, partly to make the voices of his characters sound more realistic and natural and partly to avoid the kind of utter predictability that is found in the less inventive playwrights of the period.

Regular, predictable lines of iambic meter do occur, as when the Duchess says to Antonio,

I thank you, gentle love:
And ’cause you shall not come to me in debt . . . (1.3.164-65)
More typical, however, are the immediately ensuing lines:
Being now my steward, here upon your lips
I sign your Quietus est.This you should have begged now . . .  (1.3.66-67)