This article about a “break” in the Sagittarius arm gets into direct measurements of distances a bit. Basically we can’t measure everything, but given what we can see and the measurements we can make (especially with the Gaia mission), we can work out the spiral arm structure nearby and infer the larger structure based on what we see in other galaxies.

https://www.nasa.gov/feature/jpl/astronomers-find-a-break-in-one-of-the-milky-way-s-spiral-arms

In the 1950s, a team of astronomers made rough distance measurements to some of the stars in these nebulae and were able to infer the existence of the Sagittarius Arm. Their work provided some of the first evidence of our galaxy’s spiral structure.

“Distances are among the most difficult things to measure in astronomy,” said co-author Alberto Krone-Martins, an astrophysicist and lecturer in informatics at the University of California, Irvine and a member of the Gaia Data Processing and Analysis Consortium (DPAC). “It is only the recent, direct distance measurements from Gaia that make the geometry of this new structure so apparent.”

Also take a look at https://en.m.wikipedia.org/wiki/Stellar_kinematics