Add a "required" CSS class for inputs that are required using validates_presence_of

form_builder.rb

# Adds a required css class to the label if the field
# is required in the model using a presence validator

# You need to place this in a file that will be autoloaded
# or required manually into your project. The initializers
# folder is autoloaded, but you might already have a better 
# place where you load in similar things for your project.

# Add this to your css:
#
# label.required:after {
#   padding-left: 2px;
#   font-size: 14px;
#   content: "*";
#   color: #C00;
# }

module ActionView
  module Helpers
    class FormBuilder
      def label(method, text = nil, options = {}, &block)
        classes = []
        classes << "required" if has_presence_validator? object, method

        options.reverse_merge! class: classes
        @template.label(@object_name, method, text, objectify_options(options), &block)
      end

      private

        def has_presence_validator? object, method
          has_validator? object, method, ActiveRecord::Validations::PresenceValidator
        end

        def has_validator? object, method, validator
          validator = object.class.validators_on(method).detect do |v|
            v.is_a? validator
          end

          if validator
            context = object.persisted? ? :update : :create
            validator.options.empty? || validator.options[:on] == context
          end
        end
    end
  end
end