In Stargate SG-1 episode 2.01 The Serpent's Lair, Daniel dials off the ship to the Alpha Site, and later explains that he was close enough to Earth to use Earth as the point of origin. But how does he do that exactly? It's not like the DHD on the ship would have an Earth point of origin symbol (sun over a pyramid).
It is established in several episodes, notably "Sacrifices," that there is one unique symbol on every gate, with that symbol being the point of origin. Of course, SG-1 was stunningly inconsistent with this over the course of the series, so this is contradicted in other episodes. Still, it's te closest to a good answer the series ever provided.
Each dhd has 1 unique symbol and daniel being the person who dials the off world gates most often would see it so when he dials the alpha site to get off the ship he sees the symbol and instinctively hits that button as it is unique to that dhd