Some basics:
1.  A Krag Carbine will have a barrel exactly 22 inches (muzzle to closed-bolt face). 
2.  A solid Carbine barrel band held in place by a flat spring inletted into stock.
3.  A properly contoured muzzle crown and front-sight base brazed with subtle 'dove-tail' onto barrel.
4.  A Carbine rear sight known to have appeared on a given Carbine Model.
5.  A serial number close to a 'block' of known Carbines.  (Model 1899 marked receivers were built as Carbines).
6.  A stock that is not 'plugged' at the forearm tip.  A steel pin through the stock, in front of the barrel-band, indicates a cut-down rifle stock.

  Go to the KCA Website Main page and select the tab "Photos" to see a lot of features and details.  Start a Krag reference Library, read and study.  Poyer's $20 Book is a cheap start.

Some 1899 Carbine pics and 1896 pic to help:

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