Curious question about Mistresses

Just a question from a guy who is new to this:
When visiting a domme, do you sign anything like a safety agreement?

I mean she will be in full control of me and my balls, how am I sure, that she really stops when I say stop?

And Same question for private dates... how do you trust a girl that you just met? She could castrate me so easily or just leave me tied up anywhere.