Board Thread:Questions and Answers/@comment-33646122-20171225033136/@comment-34972921-20180426074729

Sans expresses that he knows SAVEs exist on the Neutral Run when you haven't ever SAVEd. Sans does know some about timelines, but that's not entirely confirmed. RESETs aren't seemingly known to Sans, but he notes on how you feel like you were a friend post-Pacifist, as several characters do, like Toriel already knowing your pie preference. He does know that there are more than one of him, and can tell things from facial expressions, like anger after "getting dunked on", and seeing he may be wrong after saying "you haven't even died a single time". So that's pretty much all confirmed, yet hardly anything about Gaster is confirmed and his relation to Sans is only theoretical. Alphys doesn't seem to know anything about powers and things.