When someone sticks an old horn in my hand and asks if it is legitimate, I look at three things: architecture, engraving, & patination. The achitecture, that is the design and shape of the horn, needs to be similar to other known original horns & generally follow regional characteristics for the way they are designed, carved, plugged, and painted. Also examine any period hardware (drawpull, knob, staple, etc.) to be sure it is consistent with the period and region of the horn. Look for tool marks as well; they'll tell you if a modern tool was used to build the horn. The engraving needs to be similar to the styles of a given period. For instance, F&I and Rev. War horns, generally use double line, shaded (copperplate) lettering and most art work is primative in nature, almost always avoiding the use of perspective. With patination, I look for an appropriate color (even if the horn is green) & wear patterns on the horn. If there are no red flags that jump at me from any of these catagories, the horn is probably legit. But there are still some great fakers who have passed off new horns as the real thing, so be careful when you buy an old horn for less than half of what it should be worth....
All this means is, when provenance is lacking from a horn, you need to spend some serious time examining legitimate old original horns, and doing some research to dermine if your horn is a fake. The more horns you can view and handle, the better you will be at recognizing a fake. Lee