Detonation under load - you'll hear the engine pinging. If you hear it at wide open throttle, back off immediately.
If you hear it at light part throttle acceleration, not so bad, although you still would want to correct the situation.
Too much timing at steady cruising speedy and the engine may buck a little - some people call it trailer hitching or fish biting.
If you are concerned about detonation, you can check the spark plug porcelain for shiny specks of aluminum.
