Since two ties took place the chances of pregnancy are excellent. Though you will not be able to tell until the female is around 30 days into the pregnancy. Your vet can palpate the abdomen of your female and determine if there are puppies present.
You will want to have a xray done as well to determine how many puppies to expect as well as make sure they are all healthy.
Yes, just because he was castrated does not mean he still does not have any urges. Though he can not impregnate her.
If this was her first heat, her nipples will enlarge some regardless. You will see weight gain, she may be sick to her stomach a few weeks after the tie as well. She may also become clingy and her appetite may increase around 30 days into the pregnancy as well.
At 30 days your vet can palpate the abdomen to determine if she is in fact pregnant. It is recommend you have a xray or ultrasound done to determine how many puppies you are expecting. This helps when she is in labor so you can determine if there is a problem or not and make sure she does not retain any puppy's.
Dogs are pregnant for 58-68 days. You will need to increase her food intake as she gets further along if she is pregnant. A high calcium puppy food is recommended to be fed to her about a week before she has the puppies and until they stop nursing. Walter
That is about it, while there are some pregnancy tests that can be purchased they are expensive, ineffective and can not be used until 25 days into the pregnancy. Since you have to wait regardless it is better to have the vet check at 30 days which is accurate and, inexpensive.