The serratus (serratus anterior or magnus to be specific) keep the scapula stabilized against the thoracic wall when you push forward against resistance especially.They do not connect to the humerus. If you do pullups and pushups etc. that will activate it, but not as a prime mover, more of a synergist.
To active it directly try flat pressing dumbells using only the anterior movement of the shoulder joint.
One of the ways during examination to see if the serratus anterior is weak is to bend the elbow 90 degrees, flex the shoulder and internally rotate the joint so the arm is now crooked out in front of you parallel to the floor.(the position is like a horizontal elbow strike as it is thrown out in front of you.) To practice activating the muscle stand near a wall and in the described position make a motion without turning your body as if you were going to try and touch the wall with your elbow. That's the motion you want on the bench.