Discovering the truth of a situation is really a lot like fishing. You bait your hook with curiosity, or sometimes with bribes and promises, dangle it in front of everyone you meet, and wait for someone to bite.
If you do your fishing well, you get lots of nuggets of information. The useless ones you discard, but the rest you use as bait for bigger fish. Bt-by-bit you work your way up the food chain until you catch someone who can tell you everything you want to know.
When it comes to the truth, the Doctor is an excellent fisherman.