To answer your question as you asked, "how do you know if you axle is broken"? Your car would not move if it's broken in half. It would be very easy to climb up under it to see it broken. As Jae Jae stated, most cv axles that start popping, pop because the boot broke and all the greese slings out. This causes dirt to get in the joint, lose all lubrication, and thus, wear out. If you catch a broken boot before it starts popping, it's cheaper to just replace the boot and repack with greese.