It goes like this:
• What will I know in 20 years that I don’t know now?
• How can I understand & learn that now instead of learning from mistakes over the next 20 years?
• What current beliefs might be getting in the way of this inquiry?
It probably also works for facts, but I can't be sure.
Besides life decisions, it can be useful for business decisions (usually on a shorter timescale, say 3 yrs or 5 yrs).
• First step is realizing that personal growth means that one's beliefs could be very different N years from now
• A further test: Are your current beliefs identical to those held by You-20?
- Should I take this fancy CxO job if it means 20% less time with my kids?
- Is it time to sever this old friendship?
- Should I move to California?
- Should I take a 6 month sabbatical now?