If, perhaps, the question means "How is the title made specific?" then the reader looks for how the author, O. Henry, gives supporting details that point to who in his story are the Magi and why.
Della and Jim Dillingham Young, "two foolish children" as O. Henry terms them, are actually wiser than many other people because they realize that love is more valuable than any material possession such as Della's luxurious hair and Jim's resplendent gold watch. These "foolish" children are the opposite: They are the Magi, they are the wisest. For, Della and Jim know the true meaning of Christmas, Love. Out of his love for mankind, God sent his only Son to earth and the Wise Men came to worship. In a more mundane analogy--"foolish"--Della and Jim, out of their great love, sacrifice their only valuable possessions to make their loved one happy on Christmas Day, the day that Christians celebrate God's love.
I am not sure what you mean by "specified" but perhaps you are referring to how the title is explained at the end of the story. O. Henry writes this:
The magi, as you know, were wise men – wonderfully wise men – who brought gifts to the new-born King of the Jews in the manger. They invented the art of giving Christmas presents. Being wise, their gifts were no doubt wise ones, possibly bearing the privilege of exchange in case of duplication. And here I have lamely related to you the uneventful chronicle of two foolish children in a flat who most unwisely sacrificed for each other the greatest treasures of their house. But in a last word to the wise of these days let it be said that of all who give gifts these two were the wisest. Of all who give and receive gifts, such as they are wisest. Everywhere they are wisest. They are the magi.
In this sense, he explains how Jim and Della gave the best gifts of all because each gift required a sacrifice. Christians believe that Christ gave the best gift to mankind because he, too, sacrificed his greatest possession - his life - when he died on the cross for the sins of mankind. Jim and Della sacrifice the greatest things that each of them owns - Della's hair and Jim's watch. O. Henry is explaining that the act of giving a gift in the first place is a beautiful thing. It blesses the giver and the receiver of the gift.