You can ask for precise details about the implementation or the weirdest bug they had encountered.

Great tech guys are able to explain to you complex systems quite easily; not by making them overly complex, but quite the opposite, to keep them simple, and regarding the bug you can understand the depth of troubleshooting the person went through.

I do ask questions like this. You might be surprised to learn that there are a fair number of candidates who can answer stuff like this, but really struggle to write any code at all. I'm not totally sure how to account for it.