## 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*](https://rpg.stackexchange.com/questions/74670). 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 <s>weaselly</s> 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.