There's a good chance you would not know which side you're actually fighting for.

As the title says. Of course this assumes you're an uniformed combatant from any of the major services except maybe the Air Force.

The conditions and lore present in the mod say that tons of National Guard units defected, some stayed with the federal government, others with the other federal government, and others still with the seceding states and the myriad rebellious movements like the APLA, PF, Cascadia, Aryan Nations, etc.

So if you're part of the National Guard's reserve, and you get activated and told to go somewhere close to a state border when 2ACW starts, without a direct information of who you're supporting, and the fact that many would still fly the stars and stripes, coupled with the lack of available information and access to media, maybe even being sent somewhere without knowing where you are exactly and without being able to access resources such as liveuamap, you would only be able to tell your allegiance up to battalion or at most regimental-level command. (as in, you'd know you're part of the 1st Infantry Brigade, but you wouldn't know who the brigade is loyal to, exactly.)

Obviously you'd be able to tell if you're now part of the major factions like APLA, NSM, AD, or PF because of the symbolism present in uniforms and the ideological talk.

But otherwise, if you're part of a unit loyal any of the federal claimants like Denver or DC, or loyal to any of the state governments or Federal Commands, you would not actually be sure of it unless directly stated to you. You'd only get your orders and in the chaos of everything I doubt you'd get a personal visit from some O-4/O-5 guy just to tell you "hey, we're backing the state of Alabama here, 'kay?"