If you try to drive a 16d nail into it and it bends... it might be osage orange.
Seriously though, the osage fence posts I have acquired from a landowner were dark gray in color and I would have had no idea what they were other than old and hard. If you have a folding hand saw like you use for pruning limbs while hunting, you could saw just deep enough to produce some sawdust. If the dust comes out bright yellow, it is osage. You would likely only have to cut 1/4" deep, which would not compromise the strength of the post if it were not osage.