TileMap.get_cellv() will get the id of the tile you're standing on, but as you said the
id doesn't help when you're using autotile - the
id will always be the same.
Returns the coordinate (subtile column and row) of the autotile variation in the tileset. Returns a zero vector if the cell doesn't have autotiling.
Using this, you can figure out which autotile cell you're on.