// Verbose
String sObjectPrefix = '0Af';
Id sObjectId =  Id.valueOf(sObjectPrefix.rightPad(15, '0'));
SObjectType sot = sObjectId.getSObjectType();
System.debug(sot);

// Single line (thanks to Adrian Larson on SO)
System.debug(Id.valueOf('0Af'.rightPad(15, '0')).getSObjectType());