Board Thread:General Discussion/@comment-32742667-20170801135452/@comment-168424-20170801184432

I'm not sure when Aragorn speaks about knowing their identities... I think he expressed his knowledge of their origins which he could have easily learned from Elrond in their many encounters.

To add to what Tupka217 shows above:
 * FA 532: Elrond born.
 * SA 1693: Elrond may have received one of the Three Rings of Power.
 * SA 3434: Nazgûl dispersed after Sauron falls in the war of Last Alliance of Elves and Men.

Since Elrond has been involved in several of the first encounters with the Nazgûl and is known to be very learned and wise, he must know something of their origins which he probably passed on to Aragorn at some point.