you can absolutely know. they do suspiciously well. you just give harder problems until they can't solve it. how they react/approach a problem that they can't immediately solve _is_ the interview - not the "how many things they solved correctly" part.

That said - I seldom need people to be hardcore algorithm solvers What I typically did was a variation of fizzbuzz (can the candidate code very basic logic?) and then finding a bug or minor requirements extension in their online screening test/"homework" and asking them to solve that on the spot (did they write the code themselves/can they modify it). It's typically enough, there's diminishing returns to test more in-depth the programming skills - the rest you can discuss domain knowledge, general experience, working style etc.