I don't know of any
API that could do this. Best bet would be to enumerate all printers and look for a generic property (have you checked the port) that redirected printers have.
Edit: If a user redirected a printer named "Canon Bubble-Jet BJC-1000", the TS session would display the redirected printer as "Canon Bubble-Jet BJC-1000 (from %CLIENT_MACHINE_NAME%) in session %Session_ID%".
In Longhorn Server, the TS session displays "Canon Bubble-Jet BJC-1000 (%SESSION_ID%)"