He's reasonably confident he could sort little local kids for trustworthiness (not perfectly confident, kids change a lot, but reasonably so); he definitely can't sort them for aptitude. He publishes some more chemistry and biology notes - cells and molecules and circuits and so on - and wracks his brain for interesting algorithms problems that he probably overheard discussed at dinner years ago.
When he comes up with some he distributes them in housefuls of servantmakers, with a note announcing that there's a substantial monetary prize. You have a golem that can pick up rocks, one at a time, and determine their weight. How would you instruct it to proceed in order to end up with the fourth largest rock as quickly as possible?
Prime numbers are numbers that cannot be divided evenly by any whole numbers; 2, 3, 5, 7, 11, 13... Come up with a set of instructions so following the instructions produces prime numbers. What is the 10,001st prime number?
And so on. They are more computing problems than servantmaking problems but he's skimmed enough books to be confident you could solve them with the servantmaking kind of program-writing if you were very motivated.
...might be hard for them to be that motivated. He picks up extra shifts, doubles the monetary prize.