It depends on the GDS (also referred to as CRS) your travel agency uses:
For Sabre agencies:
http://www.virtuallythere.com
Apollo/Galileo:
http://www.viewtrip.com
Worldspan:
http://www.mytripandmore.com
You'll need to enter the record locator provided by the agency; not US Airways' confirmation number (PNR). Also note that the agency has to subscribe to provide those services.