Yes, that's true. It works sometimes for me. I conjecture that it's all depends on your state of mind. One of the reasons to keep a dream journal is to begin to see common themes in your dreams, and then remind yourself, the next time you see it, you'll recognize you're dreaming.

However, I've missed so many dreamsigns, just because in dreams, I seem to be dumb as a rock. One time I was at a recurring location that shows up in many of my dreams, of which I told myself, "Next time I find myself here, I'll know I'm dreaming." I tried to push my finger through the palm of my opposite hand, a common reality check, but just like real life, it didn't work. So like a dummy, I thought to myself, "Wow, so this is a real place, and not just a place in my dreams!"

More often though, probably 95% of the time, I'll see a dreamsign and just go on, totally oblivious to it, not even questioning for a moment whether I'm actually in a dream. I hate that. It's entirely possible that I'll see blinking lights one night in a dream and not even connect them to what's happening in the real world. I'd just like to try though.