shenannergan

Usually if you've been selected for a special program you'll know. Assignment to the Old Guard isn't something your DS's will recommend for you, but something like RASP, Airborne, or selection (potentially) is. As far as I understand Old Guard assignment is assigned just like any other assignment

Not OP but you don't 'consider' ceremonial units unless maybe you're in the band or something. When you enlist you'll enter into an MOS (In OP's case, I'm guessing infantry?) and the army will assign you somewhere based on that. That somewhere could be a line unit, a support unit, a ceremonial unit (though it's worth noting that just because the Old Guard conducts ceremonies, they're still a fully functional combat/line unit), or anything in between.