Take the plane to altitude, and fly it straight and level. Cut the power. What does the model do? If it tends to balloon up, that is, pitch upward, then you have too much downthrust. You'll also likely notice that the model will tend to pitch down if you go from a low-powered level flight to a high-powered level flight.

When the model's flying at high power in level flight, and you chop the throttle, it should tend to "coast" level for a bit and then gradually pitch down to maintain airspeed. If you're in low-power level flight and you abruptly add throttle, the model should start to accelerate forward and gradually pitch upward into a climb.