In the classic tales, Sir Arthur's Kight, Galahad, (sometimes Lancelot, and sometimes all the Knights of the Round Table) see an ethereal vision of the Grail. This usually provides its location. Typically, three of the Knights go on the quest, which this tale was strongly based on. Here, however, one of the Knights was named Sir Richard, and the other two were his brothers, separating it from Aurthur Grail lore.
I always assumed these brothers would be guided by visions. Namely where the Grail is, and where it should rest.
I do wish that the Knight was Sir. Galahad of the original legend, as that would have been really cool. For whatever reason, Spielberg changed the names of the Knights. If they were Round Table Knights, Merlin also has visions which help them find the Grail.