Boo wrote:

The question is as the title : given that we see it edge on, then how is it
known that the Milky Way is a spiral galaxy with arms and all ?

The most convincing direct observations are from radio astronomy using
spectroscopy of the neutral hydrogen 21cm spectral line. The amount of
material and its velocity can be deduced along a line of sight. A
short reasonably accessible intro is online at:

http://homepage.smc.edu/balm_simon/f...milky_way.html

Regards,
Martin Brown