By vetting with a proper background screening, you may be alarmed at what is revealed, or simply ensure you have an honest person in your life who you can rely on and who has your best interests at heart.  Criminal records should be clean or dated or non-existent.  Civil records may speak to character and nuance.  Personal references often reveal critical details.  The cost of having the wrong roommate affects morale, self-esteem, reputation, health, etc.  The cost of possible violations, litigation, violence, fraud and theft considerably outweigh any cost of a roommate background screening.