You can simply assume the cleverest observer, but actually it's not just about smartness (ability to compute / recognize) but also the amount of data the observ

You can simply assume the cleverest observer, but actually it's not just about smartness (ability to compute / recognize) but also the amount of data the observer has access to. For instance in a PRNG where I use some kind of source of entropy, like delay between user keystrokes, an observer that is able to watch the user typing can predict the sequence, another can not.