Many empirical sciences have special techniques, methods, or tools to make observations in systematic ways that make it likely the observations can be repeated by other people qualified to make them, thereby giving them an intersubjective and possibly objective status.

And in many empirical sciences, such as physics and chemistry, one of the basic rules that makes these sciences empirical is that experiments that cannot be repeated by others with the same kinds of observable effects have no value as evidence. Conversely, if an experiment generates observations that are new and these can be repeated by independent qualified others, the observations do count as evidence for or against a theory that implies or denies their existence.

Note that, as defined, the difference between an observation and an observable is merely that the former has been made, and the latter is supposed to be possible but has not (yet) been made.

Note also that it is not necessarily true that observations once made remain forever (presumptively) true: People may be mistaken, and they also may lie about what they have observed. The checks introduced by rules of scientific method serve to make it as rationally credible as possible that in actual fact the observations have been made and reported impartially and objectively, and can be checked and repeated by qualified persons.