How do you measure the accuracy of a stereo system?
From a more pragmatic than philosophical perspective, possibilities include:

Comparing with ground truth data--these data could come from surveying's instruments, or be assumed to be "reliable" (e.g. you could assess the system's accuracy on the Eiffel tower and other well-known buildings for which measurements have been curated);

Verifying physical properties are met (e.g., floors are horizontal, walls are perpendicular, distances between floors or between windows are constant);

Leaving one ore more views out of the 3D estimation process and assessing how far the projections of the estimated 3D points are from the actual 2D positions in these views (ie., jackknifing).