I go both ways
Join Date: Jan 2006
Bikes: Road Bikes: 2002 Litespeed Tuscany; 1996 Prototype Aluminum / MTB: 2005 SC Superlight, 1994 Trek 970
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Blow REALLY hard...
just kidding. Check the Rockshox web site, you should be able to find a manual for your Duke. Assuming it IS air adjustable, there will be a shrader valve somewhere on the fork, and you should (but don't necessarily have to) buy a fork pump. $30 / $40, with a gauge that lets you accurately check pressure.
Regardless, the manual should tell you how much air to put in for different riding conditions - I usually put in a little more (15 pounds instead of the recommend 10, for example) because I prefer a stiffer, more precise ride.