From what I've learned recruiting developers, carefully reviewing with them their work experience can already hint you a lot to what to expect in terms of abilities and proficiency. So as a long and important introduction, I would ask about their past experience.
Companies they were working for (it can be a good plus a developer understands business and what matters in a company, actually), the team size they were working in, development methodology (agile or not, if they were ticketing, how were things specified, if they had tests), systems they built (server architecture, front-end, design possibly etc.), and technology they used.
I also try to know salary expectation, even though it wasn't my task to negotiate such things. I ask primarily to make sure my technical expectations are correct. I suppose this is purely optional and hinted by the candidate seniority, though.
By the time you finish this, in addition to how related is the developer work experience, you will also know if he or she is a good communicant, if he or she took leadership roles in the past, and importantly if he or she understands the framework his or her job fits in, which is quite important when working in a startup.
You can then proceed on a coding task. We got ours from websites like LeetCode and HackerRank. I suppose you can get a task there. This task will be super easy to some and difficult to many, even not being a developer I suppose you know where to put expectation there. Some people may not pull the right algorithm on first try. Pay close attention to how they are progressing though.
Last, if the candidate is successful on the coding task, and you consider recruiting him or her, if he or she is mid-level or senior you could try to ask software engineering / whiteboard questions. This perhaps the most difficult to pull if you are not a senior developer yourself. As an attempt for this you could ask "Do you know any design pattern ?" Pick one and "Explain me that one".
If a developer has sound, related experience, knows how to code and can make a decent explanation of abstract concepts, he or she is probably worth hiring.
As Sopuli rightfully explains, you would probably prefer a good technical lead over a cheap hire in your situation, but company life being what it is, I also know many startups that started with giving job to interns, and others who struggled to get the right candidates to apply, which is why I give you a general how-to-hire for a broad range of profiles. Skipping coding test is fine for experienced candidates.