Summarize the following:
Be gentle to avoid damaging the components. A little rattling is normal. If it sounds very loud, or you notice a loose piece moving around inside the tube, then your tube is broken and needs to be replaced. Plug the tube into your amplifier or other machine. Bring along a pencil, chopsticks, or another wooden or plastic implement. Use it to gently tap each tube. All tubes ring, but the bad ones sound louder and may cause the device to squeal. Turn off the machine, then swap the tubes. Turn the knobs on your amplifier or other device to activate each tube. Tap the tubes again as you go and listen for the ringing. A bad tube will sound loud no matter what channel it's on. Swapping the old tube with a new one is also a good test. If the old tube is broken, the new one won't ring as much. Slip an oven mitt over your hand. Hold onto the suspect tube as you use your device, such as by playing a note on the guitar hooked up to the vacuum tube amplifier. You will notice much less rattling if the tube is broken.

Summary:
Shake the tube for signs of rattling. Tap the tubes with a pencil to listen to their ringing. Swap the tubes to make sure you found the broken one. Hold the tube still while using the device to test it.