Start with your state's version of Uniform Commercial Code (UCC) section 3-110 (the "model" version of that section can be read at
http://www.law.cornell.edu/ucc/3/article3.htm#s3-110).
When multiple payees are listed "alternatively" (the language in subsection (d) of 3-110), they are joined by the word "or" as in "John Doe or Jane Smith" or "John or Jane Doe." Any one of the payees may indorse and negotiate the check.
When multiple payees are named "not alternatively," they are joined by the word "and" or by an ampersand ("&"). To be negotiated, the check must be indorsed by all the payees. A check payable to "John and Jane Smith" requires indorsements by both John Smith and Jane Smith"; a check payable to "John Smith and ABC Mortgage Company" must be indorsed by both John Smith and ABC Mortgage Company.
When "and/or" or simply "/" appears between the payee names, the payee designation is said to be "ambiguous," and you treat the check as if the payees are names alternatively (using "or").
When there is no conjunction or punctuation between the payees listed:
John Smith
Jane Smith
ABC Mortgage Company
.... the courts in most states treat the payees as being listed ambiguously, and any one payee may indorse and negotiate. Check with legal counsel familiar with the court rulings on this question in your state.