I just checked my original Krag from my dad and the bolt is free after the firing pin is released. Not sure if it safe to dry fire your Krag or not. But if you do not want to disassemble the bolt you could let the firing pin down on each UNPRIMED cartridge you check you check. On mine holding the trigger as I closed the bolt seemed to work.
The only reason to even check this is case rim thicknesses do have a tolerance. Plus or minus .002 inches  total maybe with the most variation within a particular batch being plus or minus .001.
If you can detect a little play with 10 or so cases you should be good to go. You just do not want to find out that 50% of the plus .010 cases do not let the bolt close or are difficult to close as this would cause undo stress and wear on the gun.