New America wears blue arm bands according to Airlords of the Ozarks.
I would imagine that both Milgov and Civgov, both claiming to be the legitimate governemnt of the US, would be loath to adopt any markings that differed from pre-war or wartime markings. There might be tactical markings that are adopted by local commanders to distinguish between vehicles from Civgov or Milgov troops - a la the "invasion stripes" on allied aircraft for the Normandy invasion or the white stripes the Pact vehicles used during the invasion of Czechoslovakia in 1968. I think that would be relatively rare, however, as many encounters between Civgov and Milgov troops would be long-range patrols, most likely on horseback or on foot.
The Polish Free Legion would use the pre-WW II Polish flag with the national coat of arms on it (
http://en.wikipedia.org/wiki/File:Fl..._Polskiej.PNG). The Communists in Poland basically outlawwed the use of that flag. The cover art of one of the Return to Europe modules (White Eagle?) has Polish partisans flying a red flag with the Polish Eagle on it.
I could see other pre-Communist flags being used throughout Eastern Europe. The Romanian revolution in 1989 featured Romanian flags with the communist coat of arms prominently cut out of the middle.
Uniforms would likely be legacy uniforms. Shoulder patches and tactical markings would be up to the GM...