Doorway pages are often easy to identify in that they have been designed primarily for search engines, not for human beings.
They incorporate at least a minimum amount of design and navigation similar to the rest of the site to provide a more human-friendly and natural appearance.
Doorway pages are also typically used for sites that maintain a blacklist of URLs known to harbor spam, such as Facebook, Tumblr and Deviantart.
The server can differentiate between bots, crawlers and human visitors based on various flags, including source IP address or user-agent.
In either case, such redirection may make your doorway pages unacceptable to search engines.