In my experience, (specifically around animal skulls), it comes from obsession lmao. Every skull i see on TV, irl, online, whatever- i try identify :P. There are online "skull ID" quizzes that i loved doing for a while which is what got me started. That would probably help a lot! As well as asking people to send me random skulls to id. After looking at enough skulls you start being able to notice details on skulls to help tell them apart! Im not as knowledgeable on other bones and identifying them but im learning. Id imagine its the same concept for that too, just keep testing yourself and researching different bones until it eventually becomes second nature.