You can never know enough to be totally safe because even if you knew everything there are too there are too many unknown external variables to deal with. However, the amount of knowledge required to be "safe" depends on the person and the environment in which they fly. Somebody flying a modern simple aircraft close to civilisation in the first world country does not have to be as knowledgable as someone operating an ancient flying wreck for a bunch of shysters in the third world. Luck can replace knowledge only for so long.