Any air leakage will adversely affect the clarity of the tone production. Test the low and high registers of the instrument. Listen for accuracy of pitch changes. Press every chrome-plated key to be sure it moves with ease. Feel the tension or lack of it when pressing these keys.
Springs are adjustable to keep keys open or closed.
This is drawn through the instrument to keep it dry. Use the best reeds for performances. Buy a good reed container to fit into the clarinet case. Invest in a reed trimmer. This will extend the life of a reed significantly.