I was a USAF weather guesser for 12 years in the Guard. I got a side job with Midwest Weather in 2005 taking obs at a contract site. I got hired through networking with the senior observer at the site. After I was hired I was able to take the NWS test. When I took the test I was on top of my observing game, just getting back from Iraq taking manual obs for 6 months. I thought I would coast through the test...ended up passing with an 80%.
Biggest thing is getting to know the senior observer or contract holder then working your way in. From my understanding you cannot just go take the NWS obs test without employment already lined up.