1997 Skoda Felicia Cylinder Head Gasket

Hi

I need some information about my skoda.
Recently I went to make my Vrt and the mechanic noticed that may be I am going to have a cylinder Head gasket damage.he also told me that he saw a bit of oil in the reserve tank.Please can you let me know how I can know if it is really and if it is difficult to change it. Before I bought this car, the first owner put windscreen wiper fluid in the raidiator tank by mistake.Could that this type of oil that he saw is this wiperfluid.

Thanks very much and I wait for your anwer

Rube