Can prospective employer verifiy salary at current employer as well as dates?

I have heard it is true and untrue.  What does the law say?  If ok,
statistically does it occur?

Questions about law are usually not answerable unless we know where
you are. Laws vary from place to place.

Location is state of CT.

Thanks.

Simple, call the employer and ask.

Employers traditionally give each other the courtesy of providing the following:
Length of employment (Dates from when to when), the position you
occupied and if there were any promotions, the salary you were making
when you left the company. If you have not yet left the company the
salary is customarily left out. This information must be given out by
the personnel or human resources department

To prevent liability exposure, most large companies have policies that
prevent your direct supervisor from providing positive or negative
feedback on your performance as an employee to any potential employer.