They are checking for blood or mucus, but the main thing they are concerned with is the presence of white blook cells which would indicate infection. However, I have found conflicting information on the best way to capture the sample. My doctor says mid stream is fine and the Dr Hansfield and other articles I've read say it should be the first catch, as bacteria may get cleaned out by mid stream.
As Vance said, NSU is maybe. It's a difficult thing if you test negative for Gono and Chlamidya and there is no presence of white blood cells. At that point it seems to be trial and error with antibiotics.
If your concerned, go get the standard tests done.
A urinaealysis might be able to find some type of infection but also might not. NSU...maybe. Trich probablty not.