Extract 3D seam coordinates from
A seam can be defined as a line along the mesh where the vertices are doubled each with different texture coordinates.

So that's what you should look for. The set of vertices where there is a twin in the same position with different texture coordinates.

Depending on how the obj file was built that will be simple if they didn't duplicate the position when a vertex became part of a seam. Or it's not as simple because you need to deduplicate positions again.