1. Job title.
2. Date range worked.
3. Whether the employee is eligible for rehire at your company. Note that the categorization of “eligible for rehire” comes down to whether you terminate for-cause or at-will. 

Providing any other info in a reference check puts YOU at legal risk.

There is one exception: the case of public safety. 

If you fire someone for, and have evidence of, something like sexual assault/harassment but the victim chooses not to press charges, you would fire them for-cause and have an obligation to share that info. BUT MAKE SURE YOU HAVE PROOF! The same applies to situations like workplace violence, theft, fraud, etc. Even though it’s not necessarily the employer's place to press charges, you absolutely need to share the information with prospective new employers in the interest of public safety.

And by the way, this isn't part of any official regulation, it's about case law. Ex-employees are finding new and creative ways to sue their past employers, so it becomes legally risky to provide info because it's easy to accidentally "defame" an ex-employee and kill their ability to earn a living. Even innocuous statements can pass along your not-so-secret opinion, rather than fact, resulting in "boss-blocking." 

All that being said, the stars really do need to align before you'll ACTUALLY get in trouble with this. You would need all of the following:

1. You say something ill-advised during a reference check.
2. What you said prevents an ex-employee from getting a new job. 
3. The employee is told/figures out you said the ill-advised thing that cost them the job. 
4. They have to know they can sue you over it.
5. They have to take the time and effort to do so.

On the flipside, what if you're the prospective employer and you want to learn as much as you can about the applicant? As I said, you can ask the ex-employer whatever you want, but if they're wise they won't tell you much. As another source, you can ask the applicant for PERSONAL references. THEY can say whatever they want (much like the applicant can) and their information is fair game... though it's up to you whether you believe them or not.

If you want to verify the employee's skills, and whether they actually did the work they claimed at the previous job, we recommend setting up a skills test as part of your interview process. Find out more about what they are actually capable of before you invest in their hiring, without relying on a former employer to confirm.