This largely amounts to the question: "How do you tell if someone is extremely conscientious or just puts on a show of being so?"
The best answer is, wait a couple decades. Of course, that's not the answer you want, you want to know now. Close code reviews (plural) may tell. Ratios of kinds of bugs may also be a tell (if they're committing more dumb syntactic mistakes that debuggers easily catch they're probably good, and spending their time preventing ugly and subtle logic errors.)
But to the extent that algorithm choice and design is part of their job, that may not be enough.
The best answer is, wait a couple decades. Of course, that's not the answer you want, you want to know now. Close code reviews (plural) may tell. Ratios of kinds of bugs may also be a tell (if they're committing more dumb syntactic mistakes that debuggers easily catch they're probably good, and spending their time preventing ugly and subtle logic errors.)
But to the extent that algorithm choice and design is part of their job, that may not be enough.