Most every major war/near war have their own tell-tales of imminent action. This depends on tech level and environment, as Arabian cavalry is different than armoured cavalry, which is different from grav cavalry. But that’s the obvious part: there are other signs for those with eyes to see, from changes in radio traffic to a shift in tone of the leading blogs, newspapers, or bardic songs.
It could be productive for Referees to draw up a quick’n’dirty list of what to expect as the worlds the PCs frequent shifts from a peacetime to a wartime footing. Then, put the shifts in the game, and see if the PCs notice.
One thing PCs need to watch out for is increased xenophobia, on the part of local authority, the population, or both. They may need to struggle to put themselves in a neutral position, which could save their skins… or merely make them a target for everyone.
Cargo will be more tightly inspected, the police more suspicious and hostile, and traditional rights suspended. If the PCs are seen to be either “sympathetic to the enemy” or even “an enemy race”…
*Waves to Solomani, playing within the Imperium during the Rim War*
*Waves to non-Solomani, playing within the Solomani Confederation during the Rim War*
…there are going to be problems.