I basically just ask questions about hash tables; starting from the concept of associative arrays, down to the implementation of a table specific to a purpose (a minimal perfect hash with a rolling hash function). Generally if a person is trying to be a SWE and doesn't know about associative arrays, they're not going to work out. I don't expect people to answer deep questions about specific hash performance you'd get from a CS grad who studied Knuth; I want to know how you think about a mildly complicated data structure.

Along the way I ask a few questions about bits, which provides some dynamic signal (for example, would this person be good at low-level stuff?)