You can ask them anything you like ...what they will ANSWER is a totally different story.
I typically find that mom and pop / small businesses will answer anything you ask: time on the job, salary / hourly rate, is this person a good employee, etc. The big places with full-time HR departments are a bit more cagy.
What I usually do is get the two most recent pay stubs or bank statements, then call the supervisor/HR person. They will almost always tell you whether or not Mr. SMith works for them. If they refuse to talk about salary/hourly, I change tactics, like this:
"Mr. Smith says he has been a widget maker employed with your company 3 years. Let's not talk about Mr. Smith right now. Can you tell me what the average pay is for a widget maker at your company who has been on the job 3 years?"
I've had well over half of the professional HR folks answer this question for me since we are not specifically talking about a person, just getting a general feel if the salary listed on the application is accurate and I'm not holding a faked pay stub. If the HR person refuses to budge, I throw it back on the applicant to tell someone at their office to provide me the info I need to be comfy that the information is accurate.
--108.250.xxx.xxx