How do I find the topmost row of a single table, splitted among several frames in each frame.

You can iterate through all cells in a row column* and ask the first insertion point of a text cell about its parentTextFrames[0].id.

If the ID value has changed, the cell is in a different text frame. And so is its row.

In CC-2015 it could be a bit different, because we have a new type of cell: CellTypeEnum.GRAPHIC_TYPE_CELL (no insertionPoints to lead the way).

Alternatively:

Duplicate all text frames of the story the table is sitting in and check for the single tables in the duplicates.

Uwe

*edit