It should have a label on the back/bottom, with info as to the registration number and the OS installed, Pro/Home etc. It may say an earlier version, such as Windows 7 (Most likely!), but will give you the correct model (Pro etc.
I am sitting at the same Laptop at this moment. It originally came as Windows 7 Home (OEM), but I have followed through the upgrade process and it went up to Windows 8 Home. I then got a legal licence for Pro, which is what I am using now.