Ask them
Are you now, or have you ever been, a user of mind blank?
Interestingly, the spell zone of truth isn't divination - it's enchantment and doesn't seem to be affected by mind blank. This puts it an interesting position where it doesn't fall under the divination umbrella - and it also isn't giving information about the target. It is instead interpreting what they are saying: an external response analysis.
Once they're in the zone, ask them directly if they are under the effects of the spell mind blank. And yes, it's still possible for a sufficiently weaselly clever DM to come up with a non-lie response that doesn't say, but it's a pretty solid path to finding out.
This is a fairly straightforward and very unsubtle method, so its actual effectiveness may be pretty limited.
