When an order status is PartialFilled.. eg. ordered 100 but only filled 70. How can I obtain the amount filled?

I understand, if I canceled the order, I keep the 70 that was filled. Due to lag, additional 10 may be filled before the order is canceled. Now I have 80. How do can I know the filled quantity now is 80? Can I obtain this using the order ticket? I assume the order ticket is still valid even after the order is canceled.