object.respond_to?("each")

respond_to.rb

# Say hi to everybody
def say_hi
  if @names.nil?
    puts "..."
  elsif @names.respond_to?("each")
    # @names is a list of some kind, iterate!
    @names.each do |name|
      puts "Hello #{name}!"
    end
  else
    puts "Hello #{@names}!"
  end
end

%w(1,2,3,4,5).respond_to?('each')
#=> true