Sample Code:

//Create Product
Product2 pro = new Product2(Name = 'iPhone X', Family = 'Mobile');
Insert pro;

//Instantiate the Pricebook2 record with StandardPricebookId
Pricebook2 standardPricebook = new Pricebook2(
    Id = Test.getStandardPricebookId(),
    IsActive = true
);

//Execute an update DML on the Pricebook2 record, to make IsStandard to true
Update standardPricebook;

//Query for the Pricebook2 record, to check IsStandard field
standardPricebook = [SELECT Id, IsStandard FROM Pricebook2 WHERE Id = :standardPricebook.Id];
//It should return true
System.assertEquals(true, standardPricebook.IsStandard);


//Create the PricebookEntry
PricebookEntry pbe = new PricebookEntry(
    Pricebook2Id = standardPricebook.Id,
    Product2Id = pro.Id,
    UnitPrice = 1020,
    IsActive = true
);
Insert pbe;

//Query the PricebookEntry record
pbe = [SELECT Id, Pricebook2.IsStandard FROM PricebookEntry];
//It should return true
System.assertEquals(true, pbe.Pricebook2.IsStandard);