If you can do more than eight repetitions with the weight you are using, increase it. This is where you will be able to really gauge your improvement. The intensity of your workout will determine your results. Once you have reached eight reps with a weight, increase the weight. Lifting should never be easy.
Variety is also key to stimulating muscle growth.
Doing the same workout day after day will lull your muscles to sleep. Change it up whenever possible by inserting new movements, changing the order of your movements, or exchanging dumbbells for a straight bar. Any of these things will help keep your gains in mass and strength from plateauing, and will help keep lifting from becoming tedious. All this work, however, will get you nowhere if your body doesn't have the resources it needs to grow and get stronger. In order to build mass effectively, your diet must be altered with an eye toward muscle growth.