Squeaks, or notes that do not sound clear indicate a possible pad or adjustment problem. It is also important to check the body of the instrument for cracks, chips, or signs of a major repair. Student clarinets are made out of a resonite plastic which can chip. Wood clarinets crack every so slightly and can be repaired, but you should avoid purchasing them. Ask private sellers if you would be allowed to take the instrument to a local repair shop for evaluation before buying.
Some saxophones are silver plated.
A good saxophone will be dent free and clean. Saxophone pads are made of leather and orange in color. Like the flute and clarinet, these pads can be replaced but are more expensive since they are made from leather. Saxophones have many guards that protect the keys from damage. Screws that come loose often and fall off hold on all of these guards.