Originally Answered: Can a potential employer ask your present employer how much you make?
I believe they can...and, I've seen it done.
The last 3 companies where I worked had a policy that said they can't give references, but HR can validate work dates and verify salary. I believe they can not just openly call to request. I think they need to have provided an offer, and I think they need to provide the figure you gave them & then HR can validate it. But, they can not go phishing for it randomly.
Also, many offer letters will have a clause in there saying it's dependent on reference checks, passing drug test and validation of work history, etc...in otherwards, that you didn't lie on your application or misrepresent yourself. Some companies will also do background checks. Companies appear to be doing this more over the last 5 yrs.
That being said, they may not check and/or they may not check all of the details.
Good luck in getting the job.
Edit: I'm just reading above response re: people could take legal action. That is precisely why most companies say they won't give references...because people could sue them for that. But, they can validate work dates, position titles & salary.
Also, most companies where you are trying to find a job have you fill out an application. And, that application asks you to sign and say that you were truthful in answers. Some also have a clause saying it's okay for them to check the info...but, they won't do that until they are about to make you an offer.