How can you tell if a voice is YA...ey?

Maybe post a thousand words or so in SYW. I'd take a look. Also, try writing a quick query. With just 200 words to describe the book, what themes emerge to you as most important? Which POV character would the query follow? I think those types of things could help you narrow it down the most.

A character graduating high school + younger characters gaining positions of power in a fantasy land sound like popular YA themes to me. Even with adults as witnesses, it seems like you'd be okay for YA. Might even help with crossover popularity in the long run!

I still like third person past in YA, even though the trend is towards first person (but I don't think that means young adults wouldn't want to read your book).

The Book Thief is considered YA, I think, and it's written from Death's perspective, so I don't think your narrator needs to be 100% sympathetic or have a completely YA voice.

The short answer is that you can't tell if your book is YA - only readers can. I'd get some experienced YA readers to beta for you and listen to what they have to say. Sounds like an excerpt wouldn't really answer your question. Good luck!

I write in first person for YA, but I have third person segments in flashbacks and prologues.

I recommend short, snappy sentences. Use paragraph spacing to your advantage, and don't make the paragraphs too long. Also, avoid running sentences. Don't have more than two commas per sentence if you do have to break it up. If you can, use a stop.

Example:

Ashleigh stepped off the train. She let out a long sigh, and pushed her hands deep into her pockets. It would have been so easy to ride it to the next stop, and then she could have avoided the entire fiasco.

But it was too late for that.

They'd all seen her, and if she bailed, she had no doubt someone would grass her up. With some resignation, and a lot of foot dragging, she joined the throng of people shoving their way through the exit.

Perfect, she thought to herself, Another day of hell.

For me, when I switched from 3rd person to 1st person, I found it a lot easier to write in a YA voice. My 3rd person writing just comes off as too much like adult fiction even with teenage protagonists. Also like natpenna said, short snappy sentences. Most of my paragraphs aren't more than 2-3 sentences and it works somehow. At least for me.

Read a lot of YA novels and you'll notice a lot of the same thing.

edit: if you have adult POVs, and they are a good chunk of the novel, I'd say you should just aim for the adult market, since YA novels rarely have adult POVs thrown in.

The only way we can judge the voice is to hear it. If you aren't comfortable with SYW, consider seeking beta readers elsewhere on the forums. Just make sure you find people who know YA.

Depends, going by stereotype or the actual subtleties and individual variances?

This isn't a quick fix, but my only suggestion is to read a bunch of recent YA books and see how your book fits as compared to them. It's such a fluid category these days and if nothing else, keeping current on the current market/trends is super helpful.

Seconding (or is that thirding?) the recommendation to read YA books. I don't think it's really about sentence structure or punctuation or anything of that sort, but rather the issues that the character is dealing with and how they see the world. For example, if a seven-year-old finds fifty bucks on the ground, the first thought they'd have is to buy candy. If a sixteen-year-old find fifty bucks on the ground, they might use it to buy an outfit they've been lusting over for the past month. If a forty-year-old single mum with three kids finds fifty bucks on the floor, maybe she'd use it to buy groceries so the kids can actually eat some real steak for once instead of the discount mince meat.

Really look at the story you're trying to tell and what you're trying to say. If the message and themes will resonate more with adults than kids, you might want to skew it more to the adult side of things. Being not-YA isn't a bad thing. It's just not YA.

just add "I was like" to every third sentence

but on a more serious note, I think current YA is trying too hard to sound Ya-ish. I open every book and see cliché "voice", which always reads like a sassy thirteen year old who has a PhD in witty observations, even with everything going wrong