This question was bothering me when i just started LDing. There were several experiments I performed to find an answer.

1) I've been trying to read a number plate on a random car outside. At least 3 experiments failed - number plates were just a bunch of random numbers

2)There was a girl I liked by never spoke to in real life. 2 times I asked her to tell me about herself. Both times I was very highly lucid. And both times she told me a bunch of random crap (for example she told me something like I'm 4'2" I like so and so, but actually she was about 5'6" and the thing she told me doesnt exist).
I've performed similar experiments with my father and my mother, they also failed.

3)Few times i've felt like I was sharing a dream with a special DC. They seemed aware that they were dreaming. So I asked them for their email adress. Needless to say that *all* adresses were fake! Once I even caught a DC lying to me in a dream. That bastard soon was running for his pathetic dream life

4) Several times I've read some books in a dream and remembered their titles. Googling them returned 0 results...