Most states use a form of common law, which is completely based on what is common practice and what is common sense. Or at least was in England hundreds of years ago...
Which is why both sides can ask the other side to substantiate their claims or provide expert testimony to justify their claims. If there is no issue, then the other side will be able to relatively easily find someone to testify to that or provide other evidence.
It's how the system is meant to work. If this is being used abusively, the judge will catch on soon enough.