Ruby provides callbacks, to, for example, know when a new method is defined (later) in a class.

Here is a list of known callbacks.

const_missing

class Object
  def self.const_missing c
    p 'missing const was', c
  end
end

or more

class Object
  class << self
    alias :const_missing_old :const_missing
    def const_missing c
      p 'const missing is', c
      const_missing_old c
    end
  end
end