Wow. Amazing. Like what kind of an answer is “if you’re inside a car you can see the car”. This doesn’t answer a damned thing!
We don’t “know” what kind of galaxy we are in because we’ve never seen it from the outside. That would be impossible with our current technology.
Instead we have made very smart evidence-based assumptions as to how our galaxy looks.
What we know is that we are definitely in a galaxy because the general structure of the universe precludes such a conclusion. The extreme vast majority of matter is condensed in structures that are very uniform, we call them galaxies, therefore we must also be in one.
On a very clear night in certain parts of the world we are able to see in the sky a large elongated collection of stars which the ancients called the Milky Way because it resembles milk flowing down a smooth surface or something along those lines. What we currently believe is that that elongated formation of stars is an arm of one of the spirals that comprises our galaxy. We are also able to look at other structures using telescopes in our galaxy for example we have determined that we have a supermassive black hole in the center as many other spiral galaxies do as well.
So to answer your question we cannot see the Milky Way galaxy as a whole only certain parts of it and we’ve been able to draw conclusions by seeing other galaxies.
All the other comments here making neurotic epitaphs and idiotic analogies about how seeing something from the inside is the same as seeing something from the outside is just dumb and pompous if you don’t know the answer don’t say anything.