If I want to give my players clues, I just let them roll an appropriate knowledge check if they have one. Then if they roll high enough (and usually there is one player who does), I give them the relevant information. This usually works.
If they do not have knowledge, just let them roll intelligence checks and tell them: "You remember..."
And if they have forgotten something, just let them roll the same knowledge check the next playing session. Chances are they do not even remember the previous one.