Coins from sets are not exactly "MS" (mint state). That designation is reserved for coins that are intended for circulation, also commonly referred to as business strikes. The business strikes are processed on high speed machines and as such take some damage in the form of bag marks as a result of their production and distribution. This makes the average MS coin from a mint roll commonly in the MS-62 to MS-63 range, with MS-65 level coins being much more scarce.
The coins from the sets (generally, though some recent sets being an exception to this rule) are handled and packaged much more carefully resulting in an average grade of 65 or 66. Depending on who is attributing the grade, the coins from the set may be called PL-65, UNC-65, or in the case recently of ICCS, MS-65 numismatic BU (NBU); the numismatic moniker being added to differentiate the coin from a regular business strike MS coin.
As far as being able to differentiate a set coin from a roll coin, there are no distinguishing marks, but upon close inspection (again, with the exception of some recent sets), the set coins will show a different finish from the business strike coins. Generally, but not universally, the set coins will be sharper, "blacker" or more mirrored when compared to the MS coin, which will have a softer mint "bloom" luster appearance.