Spelling and grammar (spag) are always a thing. I would also ask, does it make sense? Does it flow well? Are there any places where your attention wanders or something bumps you out of the essay? Are there questions you have after reading that you feel are outstanding?

If you haven't had betas for this sort of thing before, I find it's also super helpful to simply ask, what's your impression of it? How did it make you feel?

And as you and your beta settle in together, ask them how they like to work. What if anything they specialize in, what their strengths and weaknesses are. What their approach is to feedback--some betas try to be very gentle, others are very blunt. Understanding what their preference is and how it is likely to interact with your own is super helpful for avoiding accidental hurt feelings and miscommunication.