In MOST states when prospective employers are calling a former employer re: references, they typically only ask, length of employment, rate of pay and is the prospective employee eligible for rehire. Now keep in mind, there is nothing that prevents a former employer from giving praise (which I hope you would welcome) but the employer 'could' say anything that's true about you. For example, lets says you were habitually late and were written up numerous times. An employer could say that as long as it's true. For the most part, employers are reluctant to say anything anymore in fear of being sued.

I think they're really only allowed to verify your dates of employment and maybe if you're rehire-able. Unless you have them down as a reference.