What sort of questions should I be asking during an interview for a startup?

Seconding Alex's statement that 50 is large for a seed stage. I'd go as far as to say that it's excessive in almost all cases. My first concern before figuring out if it's even a good fit would be primarily financial:

• How much money was raised in the seed round? How long has it been since the raise?
• What is the burn rate? What is the current runway?
• How has hiring directly translated into solving business needs and forming the initial product? How would my joining drive this further?

With no other context I would expect a 50 person seed stage startup to lay people off due to excessive hiring in this climate. Your questioning should be thorough enough to fully rule that out. If the company is actually solid they shouldn't mind you asking these questions, and should understand your concerns. If they're cagey and take offense to these kinds of questions I would take it as indication that they know their decision making has been poor and aren't being honest with you or possibly even themselves.

If you get past the above angle of questioning I recommend figuring out a specific project that seems to be a good fit between the company needs and your own skills and personal goals. One of the main benefits of a startup is expanded scope and impact, but there's still a constraint on business needs. I touch on this in depth in the Taro session I did with Alex over the summer: https://www.jointaro.com/lesson/8hT0g5peO8fwDZmvvIFZ/session-6-maximizing-swe-growth-ownership-and-specialization-at-startups/

50 people is pretty large for a seed-stage startup, interesting. Seed stage is generally 25 people or less.

... what sorts of questions should I make sure to ask during the interview process to make sure there's no hidden problems/wrong expectations?

I recommend this other thread as well to see if a startup has real legs: "How to evaluate a startup?"

When it comes to startup interviews, expect them to be far scrappier and intimate as the people interviewing you are almost certainly going to be working with you as your immediate teammates if you were to join. Live coding is very common for startups (e.g. I had people build a small Android app when they interviewed for my team at Course Hero), and startup interviewers take the behavioral round extremely seriously.

Passion for the mission is also critical for a seed stage startup, so do a lot of research on their product vision beforehand (and try actually using the product yourself if you can).

Lastly, since you seem to be around the senior level, here's a great discussion around evaluating if a team has senior scope: "What questions can I ask a potential new team?"

One thing to understand is the process through which they get things done. 50 people is large enough where not everyone knows all the projects going on, but it's likely too small/new to have a bunch of mature processes.

Also, get a sense of the financials. Understand how they actually make money.