I've seen enough hentai to see where this is going: The Discussion.

[Mighty Joe]

So, I dunno if this belongs in the games section, or where it would belong at all. But I thought I'd ask the forum something stupid. So, When watching hentai are you sure you are watching hentai. Imagine if you weren't actively seeking it out, what would be your signal that you're watching  hentai instead of an anime without them actually having sex.

For me, and mostly a more recent thing. The main character has no face? They just draw him without eyes and long bangs to make him a self insert more easily or something? Regardless, I thought it was always kinda funny.

[SciGirl]

A lot of times the animation quality is much worse, too. But that's not a definitive answer either. There's also title cards. But another huge tip off is lazy info dumping cue cards that explain the "plot" for people who are interested but are skippable for people who don't care.

[Mighty Joe]

True, those title cards are pretty explanatory sometimes, just a paragraph for a title.

[Foxtrot Deactivated]

Any one where all the guys apart from the main character have no facial features. Even his friends. So you that's the guy to watch.

[Mighty Joe]

Sometimes I feel you can tell just by the breast size and jiggle depending on the animation, it's not a dead give away though.

[Foxtrot Deactivated]

That's true. I've been tricked before thinking I was watching hentai and then it's a regular anime.

[Eros Angel]

I feel like the second I got the vibe someone was going to be forced into sex but end up loving it the Hentai alarm bells would go of in my head.

Not completely accurate though, have you ever seen Gundam Wing? The relationship between Heero Yuy and Relena Peacecraft, especially in the beginning, is pretty terrifying, but that doesn't go anywhere sexual.

Actually, it feels far more messed up for not being a Hentai.