Tool

Evidence-gap checker

Say what you are trying to establish. This gives you the sources that would establish it, in the order to collect them, with the ones that expire first at the top, and the source people commonly reach for that does not actually settle the question.

Pick a question above. Nothing here is a substitute for knowing your estate: it is the list you would want somebody to hand you before you start, so the volatile sources are not the ones you get to last.