Well, I found quite a reliable dreamsign for me. It is my computer being hacked, or something goes completely wrong with it and it is destroyed, or it just acts plain weird. The strange thing is how I came to the conclusion that this is a dream sign of mine.

I had this dream in which a movie played on my PC, of a person who announced he hacked the entire country of Belgium. Don't remember much of it, but I do remember going downstairs after that, and I saw my dad's bed in my brother's room. I thought nothing of it at first, but then I started thinking, that's not right! Lucidity was triggered, and shortly after I awoke. I did a reality check anyway... And it was an FA. Then I awoke again, this time for real.
I was awake for a very short period, maybe half a minute. Sleep came very quickly, and I found myself starting at the computer screen again. This time, a movie was playing with a guy who did dropkicks towards the screen. I thought "My computer again? It's almost a repetitive theme, maybe it is my dream sign. Wait, if it is my dream sign, then I am in a dream right now!" The guy on my screen starts getting closer, kicking towards me. The dream is fuzzy, I can't think straight, and I remember trying to click him with the mouse to stop him, but he reached me and I awoke.

So, I managed to determine my dream sign inside a dream, and use it to become lucid that very same dream. It just never ceases to amaze me how things inside dreams can work out.