Getting type of Class through field from another class in C#

I have a class which has a method that receives a variable:

class MyClass
{
    void ReceiveVariable(object someVariable)
    {
       //wanna get the type of the class that is declaring someVariable here
    }
}

I know this variable is a field from another class(which I don't know of), such as:

class DontKnowThisType
{
    public object someVariable;
}

How can I get the type of the class that is declaring someVariable(in this case DontKnowThisType) in my method ReceiveVariable?