PLANS & PRICING
I would consider 3 things: the team, the product and the company's mission.
If you join an early-stage startups, chances are that just a few employees will be working with you. So you need to make sure that you will enjoy staying around them.
For example, to clarify this point, you could ask if they have any interesting anecdotes about some unexpected emergencies and how they managed to fix these emergencies or even funny stories that happened in an office or after work.
You also need to know if you really appreciate the product you will be working on. In an early-stage startup, all employees (should) focus on a single product. There are no other projects you will be able to work on.
For example, to clarify this point, you could ask questions about their year and feature roadmap to understand how the product will evolve overtime.
Finally, you should make sure that you understand the company's mission perfectly. It is crucial in a startup that all employees are aligned on the company's mission. Overtime, the product might change a lot, or even pivot, but the company's mission should stay. Having all employees aligned with it gives the advantage to iterate very fast on the company's product in order to find product-market fit as soon as possible.
Hope it helps!