If we have an employee who has been off for almost month following a surgery, what can we legally ask of an employee regarding their leave?

There is no right to medical leave (e.g. leave following surgery) unless you and your employee are both covered by the Family and Medical Leave Act. Your company is covered if you have at least 50 employees who work at locations within a 75-mile radius. If you are covered, the employee must also be eligible: he or she must have worked there at least one year pre-leave, and must have worked at least 1,250 hours in the past month.
If either your company or your employee are not coverd by FMLA, you did not have to give him or her leave--the only time he or she could take would be time covered by PTO he or she had earned (if any). Once he or she exceeded the amount of PTO he or she had, you could terminate him or her--or ask whatever questions you like about return date, then terminate him/her if you don't like the answers or he/she does not honor what he/she told you.
If FMLA leave was available, bear in mind it's only up to 12 weeks. In this event (FMLA) you can't ask the employee about his/her projected return date...but if he/she stays out longer than 12 weeks, you can terminate him/her then.
You can't ask about limitations proactively: you have to wait until an employee who does return to work asks for accomodations. If/when the employee does, you can then ask for a doctor to verify the needed accommodations.