How can I determine the center of a village?

There are two village centers to consider here; one on creation, and the one you actually care about.

On world generation, the village's well is considered the center (or starting point) of the village. It is the first structure placed, and the entire village construction is based upon the well's location.

The center of the village you actually care about is the centroid of all the doors in the immediate vicinity. If you took all the coordinates of all the doors in your village and averaged them together, this would be the area where golems would spawn.

For additional details on the logic of the center of the village, read this section in the Minecraft Wiki.