A friend of mine inherited a Krag that's been ruined...I mean sporterized.

He didn't know what it was, so he showed it to me.  I was able to tell him what it is (was), but I really don't know much else about them.

What's something like this worth?  The stock has some intricate carvings in it and it's got a really old scope mounted to it.

I looked for some kind of markings on it, but they either don't have any to begin with, or someone scrubbed them all off.  

The other problem is there's no way of telling wat caliber it is.  Can a gunsmith somehow figure this out?

I'll try to get some pics of it.  It's really a shame it's been altered, it's got a hell of a smooth action.

Most of them have been sportarized over the years.

Yes, a gunsmith could figure it out with a chamber casting, or probably a couple other ways. Krags come in about 3 flavors (if memory serves)--US, Danish, and Norwegian.
Original calibers were .30-40 Krag in the US, an 8x something or other Rimmed in the Danish, and 6.5x55 Swede in the Norwegian Krags. Most common you'll see here are US Krags, and few get modified from the original calibers. I have heard of some being converted to .444 Marlin, because it just barely fits inside the magazine assembly (with a little bit of opening up). I'm still kicking myself for passing up on the Norwegian Krag I didn't buy years ago (no stock, all metal present, but had been sporterized). A Krag of some variety (preferrably unchopped) is still on my list of "to gets."
As far as worth, that is dependant on condition, whether it could be returned to original configuration, is it an heirloom, etc.?  Sight unseen for a sporterized US Krag in .30-40, maybe worth about $300-450ish to the right person. If I'm totally off, someone will come in and correct me I'm sure.

Probably 30-40 Krag.  Great rifle.  Many were sold off by the military and sporterized.  This had the effect of keeping them in good shape.  Unaltered once usually have rough bores from decades of neglect.  Just work the bolt once and you will fall in love with it.  IMHO the smoothest operating bolt action rifle ever made.  The bolt feels like it is riding on ball bearings.  These are usually pretty accurate and make great deer rifles.