I am using imgkit to take snapshot of my webpage. I run:
RAILS_ENV=production bundle exec rake assets:precompile To precompile my assets.         
        
You can use Sprockets::Rails::Helper#asset_digest_path, found here. Since you are in the controller, you have access to it with
self.class.helpers.asset_digest_path('application.css')
# => "application-7a23a105125768e41d9d24aee4553615.css"
Similarly, asset_path will yield the path to the application.css file
self.class.helpers.asset_path('application.css')
# => "/assets/application-7a23a105125768e41d9d24aee4553615.css"