Is there any such thing as a half dragon? If yes, how do you know?

I'm not sure exactly what you mean by "half dragon", but there are some fictional creatures (particularly in video games) that could qualify as part dragon and part human/elf/etc.

The "Neverwinter Nights" series (and some other Advanced Dungeons and Dragons games) contains the class "Red Dragon Disciple", which is a class that gives the character some typical dragon abilities (e.g. the ability to breathe fire). In some versions (such as the original Neverwinter Nights game), when you advance in this class enough times, the character eventually grows dragon wings.

There's also the storyline of "The Elder Scrolls V: Skyrim" in which the protagonist is described as being "Dragonborn". Whenever a dragon dies near them, they are able to absorb the dragon's soul, which allows them to gain various powers.

So those are a couple of fictional "part dragon" species present in some video games. Not sure if they qualify as this "half dragon" thing that you've seen in quizzes, but I'd imagine so!

Once, I did overhear someone describe some real life species of lizard as "half dragon" too. As in, lizards that have the word "dragon" in their species name (komodo dragon, water dragon, thorny dragon, etc.). I doubt those are the types of half dragon you have seen in dragon quizzes, as the real animals are quite different from the common interpretations of mythical dragons, but I thought I'd mention that too!

yeah, dragon that can transform into humans. I got it from Requim which is a book. Check it out,its cool