The word Seinen means "youth" in Japanese. Anime and Manga are those targeting young adult male audiences is usually classified as Seinen. The female-demographic equivalent being Josei.
Dark and cynical Shōnen titles aren't unheard of but very dark works featuring excessive gore and horror elements will most likely be Seinen.
Another way to distinguish between Shōnen and Seinen is to check for furigana. The usage of furigana over the original kanji text is a common indicator that a Manga is Seinen. If there are furigana over all characters, the title is most likely targeted towards a younger audience.
Differentiating between Shōnen and Seinen Anime is a bit more complex. Stories with a more darker plot are more likely to be Seinen.
Some popular Seinen:
- Berserk
- Cowboy Bebop
- Elfen Lied
- Erased
- Ghost in the Shell
- Hellsing Ultimate
- Knights of Sidonia
- Lupin III
- Mushishi
- One-Punch Man
- Parasyte
- Terror in Resonance
- Tokyo Ghoul
- Vinland Saga
- Yamada's First Time