require 'method_args'
  class X
    def hello(foo, bar)
      nil
    end
  end
  x = X.new
  m = x.method(:hello)
  m.args  # => [:foo, :bar]

  def blah_action
    @useful = lookup_in_database(params['foo'])
    # note that params['hello'] is ignored
    render 'blah.erb'
  end

  def blah_action(foo)
    @useful = lookup_in_database(foo)
    # note that 'hello' isn't used and thus doesn't exist
    render 'blah.erb'
  end

  def foo(bar, baz = 1, bat = 2)
    ...
  end

% sudo gem install method_args

% irb -r rubygems

>> require 'method_args'
>> class X; def foo(hello, world = 2, *blah); end; end
>> method = X.new.method :foo
=> #<Method: X#foo>
>> method.required_args
=> [:hello]
>> method.optional_args
=> [:world]
>> method.splat_arg
=> :blah
>> method.args
=> [:hello, :world, :blah]

  require 'method_args'
  class X
    def hello(foo, bar)
      nil
    end
  end
  x = X.new
  meth = x.method(:hello)
  meth.argable? # => true
  meth.args # => [:foo, :bar]
  
  meth = "string".method(:split)
  meth.argable? # => false
  meth.args # => raises StandardError