Rub or light wear on the highest parts of the coins design generally. Sometimes you can see a darker area of coloration wear a coin has come into contact with fingers or rubbed against something.
Grading is very subjective and I have spent decades learning, and looking at tens of thousands of coins. I also have paid for
ANA seminars and classes on grading at least three week long classes just on grading, several on
Counterfeit Detection and a few more on general coin history and coin photography. I listen to the experts that share their knowledge and try to learn each time I see or hear something new. This is a life time of schooling - one never really graduates! Read books on grading Photograde,
ANA grading standards, NGC and PCGS also have published books on grading, and use the "photograde" website from PCGS.com to help by comparing to coins in hand against PCGS' grading sets used on their website.
I also look at as many coins as I can at auction viewing sessions, whether or not I plan to bid on them. How often does one get to look at and examine closely early copper
US coins in high grade? Auctions are a great place to get hands on experience for nothing. (It does help that I live near by Heritage Coins), there are large auctions at most every major coin show around the country, and regular auctions in LA area and NY area.
One of the biggest challenges (even for experts) is telling an AU from a mint state coin. Knowing strike characteristics of that year and mint helps on a given coin series.
Bottom line it's up to the buyer to make the final determination - regardless of what a
TPG such as ANACS, PCGS or NGC, IGC, SEGS say on a slabbed holder. Many times they will even contradict themselves. So much so there is a whole sub-business in coins of buying under-graded slabbed coins and cracking them out to resubmit in hopes of an upgrade. Makes sense when a difference of a couple of points can mean thousands of dollars in profit. (of course there is the chance you loose the same if the
TPG goes down a couple of points in the coin submitted), this business model is slowly falling off as the
TPG's get tighter about grades they give out.
Hope this rambling helps you understand it's not quite science nor is it concrete! If there was a perfect answer we would all be using the formula!

Great question, and I hope more will chime in on this one!
"Buy the Book Before You Buy the Coin" - Aaron R. Feldman - "And read it" - Me 2013!
ANA Life Member #3288 in good standing since 1981, ANS, Early American Coppers Member (EAC), Colonial Coin Collectors Club member (C4), Conder Token Collector Club member (CTCC), Civil War Token Society (CWTS) member, Liberty Seated Collectors Club (LSCC) & Numismatic Bibliomania Society member (NBS), USMex, Member in good standing, 2¢ variety collector.
See my want page:
http://goccf.com/t/140440