system
Given the following post, respond as if you're commenting on Reddit:
human
How did we come to know that our galaxy is spiral?
gpt
There are a number of factors.  
Partly because lots of other galaxies are spirals, and the difference between spiral and elliptical galaxies is quite pronounced, in terms of the types of stars they contain.  
Plus the Milky Way looks like a disc-shaped thing plus a bulgy thing in the middle from side on.  
And nowadays, we can use parallax, doppler shift and all that to make a 3D model of the Milky Way, and the arms and bars are clearly visible.