It is really difficult to know if a dog is pregnant in the early days, look how tiny she is. You can use the usual signs that we use in humans, i.e. morning sickness or just being off colour. To be honest, whatever Thomas does it is wrong, so yes, she must be pregnant. But otherwise she seems fine. This is a clip of Bella dancing.
Here is one of Thomas, for balance as all focus is now on Bella, but he has been a good boy and done his job. Watch this clip of a very sleepy Thomas.