Keeping a thorough dream journal will help. Deep consideration will also.
Something else to consider is something I read about synchronicity: It is something which after it happens you decide was not a coincidence. And if you do decide it was just a coincidence, then it is your decision whether it was a synchronistic event.
That is, whether you believe a part of a dream was symbolically representative of a future event might only be interpreted as such after the event has happened.
However, you ma also pick up cues as to what is or is not a dream of something, and base your decisions on that.