I agree with the general rules others have stated.
Here was our recent experience: We were able to park at both Coronado and Riverside on Sunday by explaining that we just wanted to check out the property, gift shop, food court, etc.! The guards were very friendly and welcomed us on in.
On the same day, we were able to park at the Beach Club, but only because we had a reservation for brunch. They checked ID and looked up the reservation details before letting us proceed. There was a sign indicating that parking was not allowed for guests or those without ADRs.