It's impossible to know that you've had a precognitive dream until the events in the dream actually unfold sometime later in real life.
When they do, you realize that you've had a premonition dream - and naturally you want to tell people about it.
Unfortunately, the people you tell may not in believe in psychic phenomena like precognitive dreaming - and may either scoff or think you're imagining things.
Here's a "timestamped" way you can prove that you had the dream in advance of the events:
Start your own online dream blog, recording as many of your dreams as you can (since you never know which ones may turn out to be premonitions).
The blog automatically timestamps all entries, so you can prove the date you actually had the dream if it later comes true.
Alternatively, if you have a really significant-feeling dream - such as a vision of a plane crash or natural disaster - you can record it at Harry Bosma's online Dream Registry.dream blog natural disaster precognitive dream premonition premonition dream premonitions psychic phenomena