Among the other reasons listed in the thread, we can take lots and lots and lots of photographs of it from our southern hemisphere, and stitch the photos together into a composite, and compare such composites with the many many many distant galaxies we can see in the sky, many of which are disk like and presented at various degrees of edge-on-ness versus face-on-ness. Although we cannot be totally certain, the Milky Way is consistent with being a barred spiral galaxy, as it looks very much like what you get when you tilt a not-quite-edge-on view of a distant barred spiral galaxy into a completely edge-on view.

Here's one of my favourite composites. Check out the 4000 x 1212 pixel version.

https://en.wikipedia.org/wiki/File:Milky_Way_Arch.jpg

And here's one of my favourite non-composites:

http://apod.nasa.gov/apod/ap150730.html

Finally, here's a small patch of sky with a lot of various disk-like galaxies at random face-on-vs-edge-on tilts.

https://en.wikipedia.org/wiki/File:Hubble_ultra_deep_field_h...

Long exposure times, carefully stabilized equipment, and even moving the equipment outside the atmosphere have been a boon in producing these types of images.