Focus on the lightning in the film. Classic film noir is typically filmed in black and white. However, newer films, like Alfred Hitchcock's "Vertigo," are also considered film noir. Film noir uses techniques such as low-key lighting, which helps the director achieve drastic light and dark contrasts that can change from scene to scene. Another classic lighting technique in film noir is shadowing. Sometimes the actor’s faces are almost completely shadowed by darkness to give an impression that they are trying to hide something. To achieve dreamlike effects, directors use camera angles like Dutch angles, low-angle shots and wide-angle shots, which make characters look larger than life or a bit distorted. Mirrors and other effects are also used to make faces look different.
Watch the characters closely. Film noir characters are very complex. Some critics and viewers would even go so far as to call them "broken heroes" with something to prove. The main character is often a hero in their own right, but usually quite damaged. Male characters tend to hold the belief that the "end justifies the means." Film noir usually have of a private eye and a sort of femme fatale. However, film noir has developed into a category that also includes a variety of strong characters, including tortured artists, insurance salesmen, crooked cops and even jealous husbands.
Consider the plot. Film noir is dark and tends to show an extremely pessimistic view of people and society. Characters end up in situations that are unwanted, yet somehow those situations are indirectly created by the characters themselves. Although film noir tends to be dark and dreary, the characters often have an endearing quality. The plot often focuses on something that we wish we could do in real life, or it portrays how average people deal with extraordinary circumstances.
Watch the film more than once. There is no way to understand a film noir the first time through. Most film noirs are full of symbolism, both obvious and subtle. As such, it takes more than one viewing to pick up on all the details. Everything done in a film noir is done for a reason, and directors expect viewers to catch subtle clues that help explain why characters do what they do and why they are placed in these situations.