I replaced the serpentine belt in my 2005 F150 a while ago because of age and mileage. In addition to the belt itself I also replaced the tensioner and idler pulleys as a complete package.
Recently I was asked by a friend how to determine when the belt needed to be replaced. She said her belt wasn't cracked or frayed and had been in for over ten years (she's always concerned that a shop is upselling her on unneeded maintenance). In this era of EPDM (ethylene propylene diene monomer) rubber as a replacement for chloroprene material it is difficult to tell by just a simple inspection. I use the OEM recommendation if published regardless of how the belt looks.
Previously if the belt did not break you were to look for any fraying, chunks missing or cracks close together to determine if it needs replacement. The EPDM rubber belts of today wear like tires and do not show visual wear. The major replacement belt manufacturers have excellent web sites and videos on belt replacement and wear. Both Gates and Dayco have small gauges to check your belts to determine if they are worn, I think the Gates one has been updated since I got mine.
With most of the replacement intervals now at 75-100K it is cheap insurance to replace the whole drive belt assembly.
I found that RockAuto had a complete Gates kit with tensioner and idler pulleys for a good price.