Yup, you can tell which devices a machine has by looking at the pictures then look for patterns. The am9519 was a new one so I had to look at the data sheet but the upd765 will always get a specify command first so you'll see 0x03 followed by two bytes. Things like motor control is a bit harder but it's usually a bit setting just before a floppy read, write or seek.
A self-test routine also helps, I've mapped most of Displaywriter's floppy controller (i.e. parts other than upd765) by running every test it has in the ROM. There's still plenty to work on in that driver until it boots actual text editor software, though.