You may want to know if an object's class includes a module because that will tell you something about the object's behavior. It is another way of asking if an object responds to a method or set of methods, assuming you know what methods the module provides.
This can be done with the Module#include?
method.
# assuming some object book of type Book that includes Rateable
> book.class
=> Book
> book.class.include?(Rateable)
=> true
# assuming some object author of type Author that doesn't include Rateable
> author.class
=> Author
> author.class.include?(Rateable)
=> false