A good way to get information is to overhear or eavesdrop. Of course, one should never reveal that one was eavesdropping. It is almost always better to blame your insights on your powers of observation. Example: “Did you hurt your knee earlier? I noticed you winced when you put all your weight on it.” It is rude to eavesdrop, but never rude to be ultra-observant; in fact, to be ultra-observant is often impressive. Usually, it is quite simple to concoct a story about small things you observe now that corroborate what you overheard earlier. I suppose that this may also heighten your powers of observation even by practicing it after the fact.

It should be noted that it is okay, on rare occasions, to reveal that you were eavesdropping. This should only be done for dramatic effect, when the effect is desired.