How can you tell if a pad is in good condition or bad?
On worn out pads the leather of the pad has turned from a tan to a very dark brown color. The leather is usually quite hard to the touch and may be quite brittle. Some older pads that have taken on a lot of water will look "bloated" because the fibers of the felt have expanded. There will often be a black ring where the tonehole touches the pad.
Using a new pad's color and softness of the leather as a reference, it is not difficult to tell when an instrument is in need of a repad. The rule of thumb in our shop is if 1/3 or more of the pads are worn out, we advise a repad instead of a "patch job" since it will save the customer money in the long run and be a more stable and reliable repair.