I have an incomplete maintenance history on my 2010 mazda3. Currently has 75k miles. No record of belt replacement. According to my Haynes manual belts are replaced when they become shiney, cracked, or slackened. The automatic tensioner shows the belt is in spec and I can't see a single crack in the ribs. But their depiction of a "shiney" belt is offset by their books being printed on news print. Is this okay to keep or is replacement needed? 
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AdamO
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I don't know what to say about Haynes, but the "proper" way to see if a serpentine belt is worn out is to check it using a gauge. It would look something like this:
There are other models out there, but this gives you a good idea how it works. Put the edge up to the grooves. If it sits above, the belt is not worn out. If the tool is flush, get a new belt. Looking to see if a belt is "shiny" won't tell you anything. As MTA was stating in comments, the contact surface of the belt is the where the ribs are ... the only place which could get shiny is the back of the belt. This doesn't tell you anything.
That said, there would be nothing wrong with prematurely changing out the serpentine belt you have now with a new one. If you have any inkling it hasn't had maintenance before, then there's no time like the present. If the one you take off is still working, then keep it around for a spare. Doesn't hurt. IMHO, it's always better to know the state of a part then to worry over it. Just basically provides you some peace of mind which is never overrated.
Pᴀᴜʟsᴛᴇʀ2
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