You have to trust the veterinary practice and the crematorium. I have a long relationship (30 years) with the crematorium we use for our patients. I trust them, especially to cremate my own animals.
For that matter, when a parent or grandparent of yours dies, how do you know you’re getting their ashes and not another person’s ashes?
At some point, you have to have some trust that the vast majority of businesses are doing what they say they are doing.