You can mainly tell if a piece is retro based on its pattern and style. The leaders of retro would probably have to be polka dots and thin stripes. For example, colored skinny jeans have a totally different look than colored flare jeans, correct? To achieve this look with literally flying "colors", never be afraid to mix it up. Just never go overboard. Find some inspiration in the pictures below.
(All pictures from Forever21.com)