Case decision if I declared a Class

Question

How could I ask R if the class I declared is my class? What I want to do is, if I have a character vector like a<-c("vector","MyClass") I want to write a case decission if that class is written by my self or from a different package?

setClass(Class = "MyClass",
         representation = representation(name = "character",
                                         type = "character"
                                         )
         )

Or how to ask if the class exists in certain namespace?