"Tell me about a time when someone under you came to you with a problem they couldn't solve. How did you solve it?"
What other questions to ask, and what aspects of their work to probe?
I would start to test coding skills. You says you are good at it and especially for a lead, they are still part of the daily job.
Ask: How do you align a team? What are the steps you go through?
Ask: You are starting a new product, what technology do you use and why?
Ask: How do you prevent bugs? (look for what process he put in place to reduce bugs).
Ask: How do you measure the team success?
Ask: Tell me about how you hire people.
Ask: How do you run you 1:1 with the people in your team?
If you want to know about what look in the answers... this will take longer to explain. Happy to help if I can.
A medical practice wouldn't ask a physician to prove he knows medicine.
You wouldn't put a veteran over-the-road truck driver through a driving test you set up in your parking lot.
By the time someone's senior enough to be a tech lead, they'll have a stream of completed projects and glowing references. Amazing how many people will not consider those but will base everything on a slipshod tech screen. Even if third graders consider themselves qualified to teach the second grade, they're not, because they're third graders. Same with programmers evaluating other programmers.
Treat candidates like they're the scarce commodity they are, and avoid the stupid questions, buy them lunch (offsite!), and you'll instantly outperform 95% of other hiring managers.
Treat them like they're fresh out of clown college and they'll rightly recognize you as a cargo cult hiring manager. The good ones will bow out or ghost immediately, and what that leaves you with is not what you want to hire.
Hiring top talent is hard, but it's 100x as hard when you're stupid.
On "they'll have stream of completed projects", not so much. "A high tide lifts all boats" and at the same time it does not tell you a lot about the candidate's responsibility in an unsuccessful product.
I met enough tech engineer "leaders" that got their job from business people because they were good at talking even if they were pretty bad at everything else.
Soft skills are VERY important as a people manager but they cannot come at the expense of business acumen, good product sense, mediocre processes and technical inadequacy.
So when something is important for the position and it is easy to test, test it.
I agree on looking at reference for other things, harder to test.
Depending on the company and position the tech lead could be responsible for any of the below
1. managing people
2. managing product
3. managing clients
4. keeping devs focused and on a schedule, even if that means doing the "wrong" thing
5. ensuring devs do the "right" thing even if schedule slips
6. being a technology expert
7. mentoring
The first step if figuring out how you get what you want( a good tech lead) is figure out what is you want(what a good tech lead means to the organization)
Specifically in your organization what would be the primary responsibilities for this tech lead?