assets.rake

namespace :assets do
  task :check => "assets:environment" do
    # If the asset pipeline isn't enabled, safely exit.
    if not Rails.application.config.assets.enabled
      exit(0)
    end
    require 'sprockets/checker'

    # Ensure that action view is loaded and the appropriate
    # sprockets hooks get executed
    _ = ActionView::Base
    assets = Sprockets::Checker.run

    if assets.any?
      puts "There are some assets not scheduled for pre-compilation:\n#{assets.join("\n")}"
      exit(1)
    end
  end
end

checker.rb

require 'sprockets/static_compiler'

module Sprockets
  class Checker < StaticCompiler
    def self.run
      new(Rails.application).check
    end

    def initialize(app)
      @env, @paths, @manifest = app.assets, app.config.assets.precompile, true
    end

    def check
      compile

      [].tap do |missing_assets|
        env.each_logical_path do |logical_path|
          missing_assets.push(logical_path) unless @manifest.has_key?(logical_path)
        end
      end
    end

    def write_manifest(manifest)
      @manifest = manifest
    end

    def write_asset(asset)
      # We dont' want to write for real.
    end
  end
end