I have not handled as many Krags as the regulars here, they probably have seen more specials and odd balls. First thing I would check if something made me suspicious is the bore size. There were some 25 caliber wildcats in the past. Check the side to side play of a new cartridge in the chamber, the one I just replaced the barrel on had a fairly large amount of side to side play, near .010 inches but members here said that was not worrisome and it did not have issues when firing factory loads or mild reloads. If you can strip the firing pin out of the bolts you can get a relative check on headspace by checking the to and fro travel of the bolt with and without a case in the chamber. Make sure each rifle gets its correct parts back.
If an unmarked rifle was chambered for something that would be dangerous to use Krag ammo I think these checks would catch it. If one had been rechamberd for a wildcat like an Ackley it would use fireformed factory cases and you would discover that when the fired case would not even come close to chambering in a unmodified gun.
And please wait till you hear from other members as they have seen it all.

Had not even thought of using a 30/06 case as a gauge. I just checked an unfired 25/06 case in an old scrap 30/40 barrel and it stood out .6 inches from the face and tried the same case in my newly chambered 35/40 and it stood out 1.0 inches. So if you do have a 25/06 unfired round it should stand proud of the barrel face by 1/2 to 1 inch. This is with no pressure just the weight of the round, otherwise you could get the case stuck. This might work with a standard 30/06 round but I have non here at the shop to check.
I went to another site this morning and it was mentioned that PO Ackley had liked to modify Krags. If you found one of these, you might not know till you fired a round and it would not come even close to going in a standard Krag chamber afterward.
A gunsmith could cast the chambers with cerrosafe or you could do it your self to be absolutely certain. Cerrosafe shrinks minutely right after it solidifies, so you can not wait to push it out. One person here found that out the hard way.

I read a few years ago about a guy who used a product named "seal and peel" to make a chamber cast.  I had used same thing on camera platforms in Iraq to keep rain and dust out of the assemblies.  It is sorta like a silicone sealer but does not stick to anything and can be removed (peeled) without any mess.

It would not be any good for measurements but it should give you a good idea of chamber shape.  Just plug the barrel in front of the chamber and fill the chamber and let it set.  Then run the rod down the muzzle and push out the plug and the seal and peel

I have not used it on any of my guns but it should work.  Remember, though, do NOT use silicone sealer.

You just cannot be too careful.

Many, many years ago, I purchased a nearly pristine M1917 Enfield, just because it was SO nice and had a very low price.

Took it to the range with some 1962 GI match (.30-'06) ammo. Shot #1 just didn't sound right and smoke came curling out of the action. That was nothing - the ejected case was badly swollen, perforated just above the rim, and showed just a hint of beginning to form a shoulder out at the mouth of the case. Some jerk had re-chambered it to some LARGE .30 cal. magnum, without bothering to mark the work.