This is a question regarding inheritance

Question

Hi friends,

Good Day!
I am a beginner and started reading c# just a few days ago. We can inherit base class members/methods using override for which we must have a virutal declaration in base class. If we want to inherit from a class which we have not created what do we do? because we don`t know whether the member has been declared virtual. Also give me some real time example of inheritance where you want to override a method which may not be declared as vitual. 

Thanks