Dear Fanny,
In the case of Sick Leave, it should be properly communicated to the respective person (HR) in writing. Every sick leave should be supported by a medical certificate; it is as per the law. In case of employees availing sick leave very frequently, then you can ask the employee to produce treatment details certified by a doctor. This way, you will have an idea of how much time the employee will take to recover.
For authentication purposes of the certificates and reports, you can cross-check with the doctor who issued the certificate. In most companies, in-house or visiting doctors are available. If this is not the case with your company, you can hire a visiting doctor to check the certificates and other documents. By doing this, you can ask the employees to first consult with the company's doctor, and if he suggests, then they can go for Sick leave.
In case of failure to produce the certificate, you can consider it as unpaid leave. If the unpaid leave exceeds 10 days a month without approval, then you can terminate the employee. It is always a policy for new hiring that the company should ask for a medical checkup of the employee to prevent these kinds of activities. If you don't have such a policy, you can amend it and ask the current employees to undergo a medical checkup (charges should be borne by the company).
To take action in this regard, you have to talk to the employee in person. Even if the same issue continues, send him/her a warning letter stating the issue and the policy against it. And you know what to do next if the employee is on the same track...