If a person brought in non-native deer and is attempting to kill off all native deer then I would imagine that they did so in the name of large antler characteristics. Although I agree with the above, native bucks under good management will grow plenty large antlers. I don't think you can distinguish between the two just by looking at them. although stocked deer probably have ear tags.
if its in the name of antler characteristics, then do like always...shoot the ones you don't like and keep the stuff you do to propagate.