It does take practice, even with reference, of course. When I'm on bus journeys or train journeys or sitting in a waiting room where I'm alone and nothing is happening I often find myself "playing" with my hands. Moving them about, posing, watching how they catch the light, basically learning to understand them without the need to draw them right at that moment. I often watch other people's hands (as discreetly as possible, I already look nuts observing my own in public XD) so I can see how they differ to mine and how they're the same...
Observing can be as useful as just drawing.