You want to get through your workout as fast as possible, but don't rush through your reps! To ensure you're not using momentum, slow down your movements to effectively target your muscles. Instead of always lifting and lowering the dumbbell at the same speed, play around with varying your speed during each stage of the move. Lift quickly (with control), then lower slowly, then lift slowly, and lower quickly. Or try pausing in the middle of lifting or lowering. Your muscles won't know what hit them.