It's pretty easy to tell that something is old or new testament based on the writing styles. What's hard is the OT has a fair number of similar writers and a lot of books. The first question I got was [redacted] (lol) which is really hard to track down as they all sound the same to me.
Edit I didn't realize everyone had the same question. So pulled out the hint.
Personally, I know Genesis/Exodus, Psalms, and the Gospels well. I can recognize Epistles on sight, but identifying the exact one is really just a guessing game. Most everything else I am not familiar with and will take me a while to guess.
I think that's why people have enjoyed it so far, because of the tradeoff of getting a verse you recognize (so you can guess the book quickly) or getting an unfamiliar verse (so you get to expand your familiarity with the Bible).
I do not know the Bible particularly well compared to many other people, and I know the New Testament better than the Old (not uncommon - its both smaller and more important to Christians).
OT is a lot harder simply because of the number of writers and how boring/similar a lot of the writing is. To be successful you'd need to have something pulled from like the pentateuch. Maybe psalms or something like Kings/Chronicles.