If this lake has feeder streams that run all year long, someone willing to hike them in the Fall would know for sure by seeing the sea of red fish. If you have 12 inchers and they haven't planted in 3 years, you have some reproduction occurring. This doesn't mean that there is enough happening to make the lake self sustaining but it usually means the fish & game can plant less or run the risk of over populating and end up with a stunted fishery.