I have an 06 a6 with about 75000. how do i check to see if i need to replace the serpentine belt. Visually and/or audibly

 

It is a inexpensive items so I just replaced mine when I replaced the A/C compressor even though the belt was still good. To see if you need to replace the belt check for cracks or a polished surface on the grooved inside part of the belt. You can see this with a flash light just looking in from the top engine compartment.