Some variation of “What is the other person’s actual objective?” Or “Why did they do that?” or “What are they actually asking me?”
I started this habit in chess where it’s always useful to ask ‘why did my opponent make their last move?’ (and then see if there are answers past the obvious one). But I’ve also found it useful in other areas. Several times at work I've gone through iterations of something with someone because I answered exactly what they said instead of what they actually wanted. I now try to stop and ask them what their actual purpose is and it often saves me a bit of work.
I wish more people would try to solve the problem rather than answer the question, especially when it takes little additional effort. example: q: "Will material x work in this scenario?" A1: "No, that's not a good choice." vs A2: "No, instead try or Y" or even A3: "No, in fact none of our products will."
See also: Boring Advice Repository, Solved Problems Repository, Grad Student Advice Repository, Useful Concepts Repository, Bad Concepts Repository
I just got back from the July CFAR workshop, where I was a guest instructor. One useful piece of rationality I started paying more attention to as a result of the workshop is the idea of useful questions to ask in various situations, particularly because I had been introduced to a new one:
"What skill am I actually training?"
This is a question that can be asked whenever you're practicing something, but more generally it can also be asked whenever you're doing something you do frequently, and it can help you notice when you're practicing a skill you weren't intending to train. Some examples of when to use this question:
Many of the lessons of the sequences can also be packaged as useful questions, like "what do I believe and why do I believe it?" and "what would I expect to see if this were true?"
I'd like to invite people to post other examples of useful questions in the comments, hopefully together with an explanation of why they're useful and some examples of when to use them. As usual, one useful question per comment for voting purposes.