Hints and clues. Be subtle, but balanced. Let them hear of someone who knows, but is hard to find or only knows bits and pieces (one bit is that the goal lies to the north, perhaps). Again, be careful of metagaming and above all, be subtle!
Ways to give hints and clues without sounding like an orge…unless the NPC is an orge.
1. Don’t use prophecies
2. Mysterious strangers…we all know Gandaf.
3. Make connections logical…this key fits this lock…in the other dungeon.
4. Don’t over explain.
5. Let them talk it out and come to a conclusion.
6. Don’t interfere with their logic sessions…sit back and marvel.
7. Be consistent in your DMing.