How is an EGR stuck open/closed code set?

[SoCal LS-L]

I have been plagued with this EGR stuck closed code since I bought it, I know its stuck closed because it runs great otherwise. I have cleaned the EGR valve in and out with solvent, verified the operation of the pintle, and cleaned the filter that sits above it. The MIL stayed off for about a week after this was done but has returned with the same code 56. With no sensor lead to the EGR valve, how does the computer determine that the valve is either stuck open or closed? How could it possibly know? If someone could answer this id be that much closer to finding the real problem.... thanks in advance!