Press m e to check their uniform (equipment) assignments. From here, you can either change individual squad-position equipment, or you can assign a whole uniform to a whole squad. You will also see green checkmarks by the equipment that has been assigned -- but this does not necessarily mean the dwarf is actually wearing it.
Press m f to check ammo assignments. A full squad of crossbow dwarves should have about 400-500 bolts assigned; the default is 250, or 25 per dwarf on average, but it's good to have more assigned due to the way they fill their quivers. (A dwarf who fires 1 bolt now has a quiver containing a stack of 24. 24 is less than the capacity of the quiver, so the dwarf will pick up the next assigned stack of ammo and add it to the quiver. A stack of bolts may be as large as 25, so this could mean the quiver now holds 24 + 25 bolts. Thus, you can have up to 490 bolts in active use by a single squad.)
If you don't use hunters, you may also want to unassign bolts from the "hunters" pseudo-squad, available on the m f screen. This frees up a bunch of bolts for use by regular military squads.
You can also see which stacks of bolts are assigned to a squad, and which are assigned to individual members of the squad (the latter have the green checkmarks).