There are no absolute rules on button attachment methods, e.g. "Parallel attachments are always done by hand, cross-stitched attachment is done by machine, crows-foot is done by hand," etc. Now there are machines capable of doing all three methods. Two-hole and even three-hole buttons can be attached by machine. Machines can easily do shanks. In general, though, a button was probably attached by hand if it is held in place by a small number of fairly tight stitches made using a thicker thread. A button was probably attached by machine if it is held in place by many loose stitches made using thin thread.  an additional note: when buttons are attached using two rows of parallel stitches and a single horizintal stitch, you can be almost certain that it was done by machine. Machines, unlike humans, seem to be unable(to date.) of crossing over to sew the second row without carrying that a stitch across.