Since the 1980s (and even before), there have been third-party grading companies which will independently assign grades to coins.  As part of their service, they will encapsulate the coin in a tamper-evident holder and also guarantee the authenticity of the coin.

There are three major grading services accepted and recognized by GreatCollections.  These are:

PCGS (Professional Coin Grading Service)
NGC (Numismatic Guaranty Corporation)
ANACS

Please, note there are literally dozens of other third-party grading services that have turned up over the past few years.  Many of these are not independent and do not offer guarantees on the services they provide.