Question about Acts

Greetings all,

I hope my question does not take the discussion two steps backward but I'll 
fire it off anyhow.

I am studying Paul's speech to the Athenians for a class project.  In Acts 
17:25 the two words "prosdeomenos tinos" are translated by the NRSV as, ". . 
. as though he needed anything. . . "  The NIV translates it, ". . .  as if 
he needed anything. . ."  Are there grammer rules that govern the 
translation of this as an 'aside' or is it taken from the context?

Thank you for your time,

James Lindberg
Luther Seminary
St. Paul, MN