No test that I'm aware of for the valve.
Did you try moving the valve (piston) full stroke to see if it moves freely?
Move the piston slowly and smoothly. Any sign of resistance or sticking, (either inward or outward) and you should replace the valve.
Here's the diagnostics for P0489:
Theory of Operation
There are two portions to this diagnostic: 1. The diagnostic monitors motor effort as a measure of current. When the valve is opening to a position, and uses too much motor effort to achieve that position the diagnostic will fail. This will most likely occur in a situation when the valve is stuck. 2. At every key on, the valve tries to recalibrate it's position. The valve shaft will move until it comes into contact with the valve. If the distance the valve shaft must move is different by a calibrated amount than the original learned movement then the diagnostic fails. This will most likely occur if the valve is stuck in an open position. The valve will perform this autozero function at every key on and every time the valve is commanded closed throughout the engine operating cycle.
The ECM will illuminate the MIL lamp after the diagnostic runs and fails in two consecutive drive cycles. The ECM will also turn on the ETC lamp. During this time the ECM will not longer control the EGR valve. The MIL lamp will turn off once the diagnostic runs and passes in 4 consecutive drive cycles.
• When Monitored: At key on, while engine running
• Set Condition: Motor effort exceeds a calibrated level or autozero function fails.
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 • ENGINE DIAGNOSIS AND TESTING)
Diagnostic Test 1. DTC P0489 ACTIVE
Ignition on, engine not running. With the scan tool, read DTC's. Is DTC P0489 active?
Yes »Replace EGR valve. Perform POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
»Refer to the INTERMITTENT CONDITION - DIESEL (Refer to 9 - ENGINE - DIAGNOSIS AND TESTING)