If you have an object by itself (let's say as an input parameter to a method with type object), with no definition or generic type, there is no way to find the type. The reason is simple, you cannot send message to (invoke any method on) the object to ask about the type.
There could be some other workarounds, as you see in some answers, like using generic types. In that case, you're not asking the Null object, you are asking the generic type for its type.
