public class MyController
{
  public MyController() {}

  public List<X__c> selectX()
  {
     if (X__c.SObjectType.getDescribe().isAccessible()) {
       if (X__c.Fld__c.getDescribe().isAccessible())
       {
         return [SELECT Id, Fld__c FROM X__c];
       }
     }

     return null;
  }
}

public class MyControllerTest
{
    @isTest
    static void verifySelectX()
    {
        // Given
        X__c[] xRecords =
            (List<X__c>) TestFactory.create(new X__c(), 2);

        // When
        xRecords = new MyController().selectX();

        // Then
        System.assertNotEquals(null, xRecords);
        System.assertEquals(2, xRecords.size());
    }

    @isTest
    static void verifySelectXNoAccess()
    {
        // Given
        X__c[] xRecords =
            (List<X__c>) TestFactory.create(new X__c(), 2);

        // Create a user which will not have access to the test object type
        User testUser = createUser('Chatter External');
        if (testUser == null) {
            // Abort the test if unable to create a user with low enough acess
            return;
        }

        // When
        System.runAs(testUser)
        {
            xRecords = new MyController().selectX();

            // Then
            System.assertEquals(null, xRecords);
        }
    }
}