If it is an 03 barrel and assuming the outside contour has not been greatly altered, it will have the very distinctive contour of an 03 or 03a3. An 03a3 with roughly be 23 or so inches long after cutting and rethreading the breech. The "breech re-enforce will be heavy then quickly taper via a radiused taper into the body of the barrel. The muzzle will show a length wise slot for the sight index key, unless it has been recontourd or cut off. An 03 will be the same but for the addition of a swelling resulting in the appearance of a ring upon the outside of the barrel just in front of the radiused taper section. This originally was a contack point for the rear sight collar. Very often Krags rebarreled with 03 /03a3 barrels will retain the 03 / 03a3 front sight.
One other thing I have noticed and it may not by any standard be universal; original Krag chambers seem the leave the front of the case shoulder appearing to have a concave radius upon firing. rebarreled chambers leave the cases appearing "normal" upon firing.
